Atomic Structure and Chemical Bonding

(a) The idea of the smallest unit of matter was given by Maharshi Kanada. According to him, matter consisted of indestructible minute particles called paramanus, now called atoms. A paramanus does not exist in free state, rather it combines with other paramanus to form a bigger particle called anu, now known as molecule. There are different types of paramanus. Each one of them exhibits specific properties.

(b) Democritus called the paramanu an atom, which is derived from the Greek word atomos, meaning indivisible.

John Dalton gave the first scientific theory about the structure of matter. He considered atoms as indivisible particles that are the fundamental building blocks of matter. The existence of different types of matter is considered to be due to different types of atoms constituting them.

Main postulates of Dalton's atomic theory are:

1. Matter consists of very small and indivisible particles called atoms.
2. Atoms can neither be created nor be destroyed.
3. The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
4. Atoms of an element combine in small numbers to form molecules.
5. Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
6. Atoms are the smallest units of matter that can take part in a chemical reaction.

An α (alpha) particle is a doubly charged helium ion (He²⁺) containing two protons and two neutrons. It is formed by removing 2 electrons from helium atom.

Cathode rays are a stream of electrons that travel from the negatively charged electrode (the cathode) to the positively charged electrode (the anode) in a discharge tube.

Formation of cathode rays — When a high voltage (10,000 volts) charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of Hg), the gases become good conductors of electricity and begin to flow from cathode to anode in the form of rays which are called cathode rays.

Cathode rays are negatively charged as they are made of negatively charged particles.

Anode rays are positively charged as they are made of positively charged particles.

When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are produced.

In discharge tube experiment, Goldstein noticed another set of rays travelling in a direction opposite to that of the cathode rays i.e., from anode towards cathode, when perforated cathode was used in the discharge tube. He called these rays as canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays.

Discovery of sub-atomic particles i.e., electrons, protons and neutrons shows that atoms are divisible.

In discharge tube experiment, Goldstein noticed, set of rays travelling in a direction opposite to that of the cathode rays. These were called canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays. These rays consist of sub-atomic particle proton.

(a) Hydrogen does not contain neutron.

(b) If an atom contains one electron and one proton, it will not carry any charge i.e., it will be a neutral atom.

According to Thomson's atomic model —

1. An atom is considered to be a sphere of uniform positive charge and electrons are embedded into it.
2. The total positive charge is equal to the total negative charge so that an atom as a whole is electrically neutral.

(a) J. J. Thomson — Electrons

(b) E.Goldstein — Protons

(c) James Chadwick — Neutrons

(a) Proton (+1)

(b) Electron (-1)

(c) Neutron (0)

(a) Rutherford selected gold for his α-particle scattering experiment as he wanted as thin a layer as possible and gold is the most malleable metal.

(b) (i) Heavy nucleus like platinum will show the same observation with α-particles as shown by gold foil.

1. Most of the α-particle will pass straight.
2. Some α-particle will slightly deflect from their straight path.
3. Very few will be deflected by very large angles or completely bounce back.

(ii) If light nuclei like lithium is used then fast moving α-particles may even push the light nucleus aside and may not be deflected back.

According to Rutherford's model of an atom, there is a positively charged mass in the centre of the atom, i.e., nucleus, in which the entire mass of the atom is concentrated, which makes nucleus the densest part of the atom. The electrically positive nature of the nucleus is supported by the fact that the positively charged α-particles approaching the nucleus get deflected. This proved that nucleus contained protons.

Rutherford discovered the nucleus.

Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particle pass through the metal foil without deviating from their path. This showed that an atom contains a large empty space (extra nuclear space).

The comparison of electrons with planets in the solar system is the main drawback of Rutherford's model. It failed to explain the stability of the atom.

(a) A is First energy level (K shell)

B is Second energy level (L shell)

C is Third energy level (M shell)

Shell K has the minimum amount of energy.

(b) X is the nucleus, it is positively charged.

(c) The above sketch is of Bohr's atomic model model of an atom.

Postulates of Bohr's atomic model are as follows

1. The electrons revolving around the nucleus are confined to certain fixed orbits called shells or energy levels, each of which is associated with a fixed amount of energy.
2. While it is revolving around the nucleus in an orbit, an electron neither loses nor gains energy.
3. An electron revolving in a particular orbit, on gaining a certain amount of energy, jumps to the next orbit and vice verse. Since each orbit is associated with a fixed amount of energy, Bohr called it an energy level.

(a) The three fundamental particles of an atom are:

1. electrons
2. protons
3. neutrons

(b) The table below shows the symbol and charge of the particles:

P — 12

Q — 17

R — ²³⁸₉₂U

S — Fluorine

Atomic number (Number of protons or number of electrons) = 4

Mass number (Number of protons + Number of neutrons) = 4 + 5 = 9

Atomic number of sodium is 11 implies that number of protons in the nucleus of sodium = number of electrons in a neutral atom of sodium = 11

Mass number of sodium is 23 implies that the sum of protons and neutrons in the nucleus of sodium = 23

p = proton

n = neutron

e = electron

Distribution of electrons into various orbits was suggested by Bohr and Bury

1. The maximum possible number of electrons in a particular shell is given by the formula 2n², where n is the serial number of that shell.
   First shell or K shell has (2 x 1²) = 2 electrons
   Second shell or L shell has (2 x 2²) = 8 electrons
   Third shell or M shell has (2 x 3²) = 18 electrons
   Fourth shell or N shell has (2 x 4²) = 32 electrons
   and so on.
2. Electrons are not accommodated in a given shell unless the inner shells are filled. In other words the shells are filled in stepwise manner.
3. The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more than 8 electrons.
4. The outermost shell of a chemically stable atom can accommodate a maximum of 8 electrons, except Hydrogen and Helium atoms, which have only one shell, and can thus accommodate a maximum of 2 electrons only.

Below is the orbital diagram of ₂₀⁴⁰Ca⁺² ion:

Three fundamental particles of ₂₀⁴⁰Ca⁺² are:

1. Protons — 20
2. Electrons — 18
3. Neutrons — 40 - 20 = 20

Mass number = 24

Atomic number = 12

No. of electrons = 24 - 12 = 12

Electronic configuration = 2, 8, 2

The atom is neutrally charged, because one electron has -1 charge and one proton has +1 charge. Both the charges will cancel each other. So, the atom is electrically neutral.

Total number of protons and neutrons present in the nucleus of an atom determine its mass.

Atomic number is equal to number of electrons.

M shell is the third energy level, K and L shell are completely filled.

The electronic configuration
K shell - 2 electrons
L shell - 8 electrons
M shell - 3 electrons

Total no. of electron = 2 + 8 + 3 = 13

Hence, total number of electron is 13. So, the atomic number of the element is 13.

The most stable atom from the given list is $^{40}_{18}\text{Ar}$ (Argon).

Reason — Argon is a noble gas with a completely filled outermost shell, making it chemically inert and highly stable.
In contrast, the other atoms, $^{23}<em>{11}\text{Na}$, $^{35}</em>{17}\text{Cl}$, $^{27}_{13}\text{Al}$, have incomplete outer shells and are therefore reactive as they tend to gain, lose, or share electrons to achieve stability.

James Chadwick discovered neutrons.

(a) Neutrons are located in nucleus of an atom.

(b) Neutrons are neutral in charge.

(c) Mass of a neutron is 1.676 x 10^-24 g.

(a) Atomic number = Number of electrons
Hence, atomic number is 9

(b) Mass number = Number of protons + Number of Neutrons
Mass number = 9 + 10 = 19
Hence, mass number is 19

(c) Electronic configuration - 2, 7
K shell - 2 electrons
L shell - 7 electrons

Given, Atomic number = 16 and mass number = 32

So, Number of protons = 16

Number of electrons = 16

Number of neutrons = 32 – 16 = 16

Electronic configuration = 2, 8, 6

Below diagram shows the arrangement of the electrons in an atom of sulphur:

(a) electronic configuration of ₁₃²⁷X is [2, 8, 3]

(b) electronic configuration of ₁₇³⁵Y is [2, 8, 7]

Number of electrons in X = 13

Mass number of Y = 35 and number of protons in Y = 17

∴ Number of neutrons in Y = 35 - 17 = 18

Atomic nucleus

Reason — Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particles passed through the metal foil without deviating from their path. Very few (nearly one in ten thousand) alpha particles were either deflected by very large angles or completely bounced back. This suggested the presence of a positively charged mass at the centre of the atom, known as the nucleus.

8

Reason — Oxygen has atomic no. 8. Its electronic configuration becomes 2, 6 i.e., it has 6 valence electrons. In O^2-, there are two more electrons. Hence, number of valence electrons in O^2- is 8.

2, 8, 8, 1

Reason — Potassium has 19 electrons hence the electronic configuration is [2, 8, 8, 1]

23

Reason — As the atom is unipositive so the actual number of electrons must be 11 and mass number will be 11 + 12 = 23

8

Reason — Chlorine has atomic number 17 and electronic configuration is 2, 8, 7. For Cl^-, number of electrons increases by 1 hence electronic configuration becomes 2, 8, 8 and valence electrons will be 8.

2, 7

Reason — Fluorine has atomic number 9 and electronic configuration is 2, 7.

³¹₁₅A

Reason — Atomic number is 15 so it will be written in subscript and mass number = 15 + 16 = 31 so it will be written in superscript. Hence the correct representation will be ³¹₁₅A

1

Reason — Number of electrons = number of protons = 3, hence electronic configuration = 2,1. Hence, the valency is 1.

Can be metals or non-metals

Reason — If the element shows positive valency it is a metal (e.g., Beryllium) and if the element shows negative valency it will be a non-metal (e.g., Sulphur).

Protons and neutrons

Reason — The nucleus is located at the centre of the atom, it contains protons and neutrons, which account for the total mass of that atom.

13

Reason — M shell has 3 electrons so the electronic configuration becomes 2, 8, 3. Hence the total number of electrons = 13 and the atomic number becomes 13.

18

Reason — The maximum possible number of electrons in a particular shell is given by the formula 2n², where n is the serial number of that shell.
Third shell or M shell has (2 x 3²) = 18 electrons

K

Reason — Orbits are arranged as K, L, M, N. So the orbit closest to the nucleus is K.

Noble gas

Reason — Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas i.e., 2 or 8 electrons in its valence shell.

Neutrons

Reason — Isotopes are the elements having the same atomic number but different mass numbers.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in terms of neutrons.

(a) Hydrogen

(b) Helium

(c) Magnesium

(d) Isotopes

(e) K shell

(a) As atomic number 15 has electronic configuration [2,8,5], so it is a non-metal having 5 electrons in the valence shell and needs three electrons to complete its octet so it has a valency 3.

(b) As atomic number is 8, so it has the electronic configuration of [2,6] and it needs 2 electrons to complete its octet hence, it is a gas of valency 2

(c) As atomic number 19 has electronic configuration [2,8,8,1], so it is a metal having 1 electron in the valence shell and donates its one electron to complete its octet so it has a valency 1.

(d) As atomic number 14 has electronic configuration [2,8,4], so it is a non-metal having 4 electrons in the valence shell and needs four electrons to complete its octet so it has a valency 4.

(a) Elements with the valence shell having 8 electrons and helium with 2 electrons, cannot gain or lose electrons so they are chemically inactive and are called inert elements.

(b) Inert elements exist as monoatoms because they have a completely filled outermost electron shell. This makes them very stable and unlikely to form chemical bonds with other atoms. Hence, they tend to exist as monoatoms in molecules rather than participating in chemical reactions.

(c) Electrons present in the outermost shell (i.e., valence shell) of an atom are valence electrons.

Hydrogen has three isotopes i.e., namely protium (¹₁H), deuterium (²₁H) and tritium (³₁H) each having the same atomic number i.e., 1 but different mass numbers i.e., 1, 2 and 3 respectively.

Hence, all three have same number of electrons and protons whereas number of neutrons are 0, 1 and 2 respectively.

Isotopes of Hydrogen

As elements are arranged on the basis of the number of protons in their respective atoms, therefore, proton helps in determining and understanding the structure and behavior of an atom. Protons also determine the positive charge of the nucleus in the atom

(a) X⁺

Reason — Since the element X has 1 electron in its valence shell. So, it will lose an electron and gain noble gas configuration. Therefore it forms a cation (X⁺). The sign is positive and the charge is of 1 proton.

(b) X will be a strong reducing agent as it has the tendency to donate its valence electron.

(a) Mass number (A) — The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons ) in it's nucleus. It is denoted by A.

A = number of protons + number of neutrons

Mass number however is a whole number approximation of the atomic mass calculated in atomic mass units.

(b) Ion — An ion is a charged particle which is formed due to the gain or loss of one or more electrons by an atom.

For example, a sodium ion (Na⁺) is formed when a sodium atom loses one electron and Chlorine ion (Cl^-) ion is formed when a chlorine atom gains an electron.

(c) Cation — A metallic atom, which loses electron(s), becomes a positively charged ion and is known as a cation.

(d) Anion — A non-metallic atom, which gains electron(s) becomes a negatively charged ion and is known as an anion.

(e) Element — An element is a pure substance that can neither be formed from, nor decomposed into simple substances by ordinary physical or chemical methods.

(f) Orbit — The imaginary circular path around the nucleus of an atom, in which the electrons revolve.

Atomic number = 2

Mass number = 4

(a) Number of electrons in the M shell = 2. So, the electrons in the K and L shells = 2 and 8

Hence, atomic number = 2 + 8 + 2 = 12

(b) As number of protons = number of electrons = 12

(a) (i) Atom E.

As its atomic number is given as 7, therefore it contains 7 protons.

(ii) Atom B.

As its atomic number is 9, hence it has 9 electrons, therefore, its electronic configuration is 2, 7.

(b) Atom C has electronic configuration : [2, 1]

Atom D has electronic configuration : [2, 6]

As D requires two electrons to complete its octet, hence, it combines with two atoms of C which donate their valence electrons to attain stable noble gas configuration and D accepts the two electrons and attains a stable state, hence the formula of the compound formed between C and D is C₂D

(c) (i) Metals: A [2,8,8,2] and C [2,1]
Reason — As the elements A and C have 2 and 1 electrons respectively in the valence shells hence, they are metals.

(ii) Non-metals: B [2,7], D [2,6], E [2,5].
Reason — As the elements B,D and E have 7,6 and 5 electrons respectively in the valence shell, hence they are non-metals.

(a) (i) The composition of their nuclei :

$^{24}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 24

As, neutron [n] = A - Z = 24 - 12 = 12.

Hence, p = 12, n = 12

$^{26}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 26

As, neutron [n] = A - Z = 26 - 12 = 14.

Hence, p = 12, n = 14

(ii) Their electronic configurations :

$^{24}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

$^{26}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

(b) Reason for difference in mass numbers — As the number of neutrons is different in the two isotopes of magnesium, therefore, they have different mass numbers.

Constituents of the nucleus are called nucleons, i.e. protons and neutrons.

³¹₁₅P has atomic number 15, hence it has 15 protons.

Mass number 31, hence number of neutrons = 31 - 15 = 16

Therefore, number of nucleons are p + n = 15 + 16 = 31.

Structure of Phosphorus is shown in the diagram below:

Isotopes are atoms of the same element having same atomic number but different mass number.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in the number of neutrons.

Uses of isotopes

1. Radioactive isotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.
2. An isotope of Uranium ²³⁵₉₂U is used as a fuel in nuclear reactors.

Isotopes have the same atomic number i.e., the same number of electrons and protons and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. Thus, the isotopes of an element are chemically alike.

Isotopes have different mass numbers [A] due to different number of neutrons. ³⁵₁₇Cl has 18 neutrons whereas ³⁷₁₇Cl has 20 neutrons. Due to different atomic masses these atoms differ in their physical properties like density, melting point, boiling point, etc.

Fractional atomic mass is the weighted average of the atomic masses of all the naturally occurring isotopes of that element.

Fractional mass of chlorine is 35.5.

Any natural sample of chlorine contains 75% of ³⁵₁₇Cl and 25% ³⁷₁₇Cl, i.e., these isotopes are in the ratio of 3:1 respectively. Thus, average atomic mass of chlorine is calculated as

$\dfrac{(35\times3) + (37\times1)}{4}$ = 35.5. Therefore, atomic mass of chlorine atom is 35.5

(a) Atomic number of an element is the number of:

• electrons present in its neutral atom.
• protons present in the nucleus of its atom.
• positive charge in the nucleus of its atom.
• it is denoted by Z.

(b) Complete the table given below:

(c) Write down the electronic configuration of

(i) Chlorine atom — 2, 8, 7

(ii) Chloride ion — 2, 8, 8

(a) Sodium atom —

(b) Chloride ion —

(c) Carbon atom —

(d) Oxide ion —

(a) Physical properties depend on the atomic mass (mass number) and isotopes have different mass numbers due to different number of neutrons hence they differ in their physical properties.

(b) Argon [2, 8, 8] does not react or do not combine either with other atoms or among themselves to form molecules, since their octet is already complete, hence, they are in a stable state and their combining capacity is zero.

(c) Actual atomic mass is greater than the mass number because the mass number is a whole number approximation of atomic mass unit. Actual atomic mass is fractional as it is the weighted average of all the naturally occurring isotopes of that element.

(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of chlorine is same, i.e., electronic configuration of ³⁵₁₇Cl is [2,8,7] and electronic configuration of ³⁷₁₇Cl is [2,8,7], hence they both have same chemical properties.

(a) Radioactive isotopes — Used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.

(b) ²³⁵U — Used as fuel in nuclear reactors.

(c) ⁶⁰Co — used in radiotherapy for treating cancer and other diseases.

(d) ¹³¹I — Used in treatment of goitre.

(e) ¹⁴C — Used for determining the age of historical and geological material.

Contradictions with Modern atomic theory:

Correlation with Modern atomic theory:

The modern atomic theory however correlates with Dalton's atomic theory in the fact that atoms are the smallest unit of matter taking part in a chemical reaction and in a given compound the relative number and kind of atoms is a constant.

(a) For an atom to achieve stable electronic configuration it must have 8 electrons in outermost orbit like that of noble gases other than Helium. This is known as Octet rule.

(b) For an atom of hydrogen to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium. So, two hydrogen atoms share one electron each to become stable. This is known as Duplet rule.

(i) Valency of each element:

Element A : Atomic number (number of electrons) = 7

Electronic configuration = 2, 5

Valency of A = 8 - 5 = 3

Element B : Electronic configuration 2, 8, 8

Valency of B = Zero

Element C : has 13 electrons

Electronic configuration = 2, 8, 3

Valency of C = 3

Element D : Protons = Electrons = 18

Electronic configuration = 2, 8, 8

Valency of D = Zero

Element E : Electronic configuration = 2, 8, 8, 1

Valency of E = 1

(ii) C and E are metals.

(iii) A is a non-metal.

(iv) B and D are inert gases.

(a) Y (2, 7) forms an anion.

(b) Z (2, 8, 2) forms a cation.

(c) X (2, 4) has four electrons in the valence shell.

Atoms attain noble gas configuration either by gaining electrons or losing electrons or sharing of electrons with other atoms of the same element or another element to form a chemical compound.

The chemical bond formed between two atoms by transfer of one or more electrons from the atom of a metallic (electropositive) element to an atom of a non-metallic (electronegative) element is called ionic bond or electrovalent bond.

Bonds formed between metals and non-metals are electrovalent bonds.

Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.

(a) Since there are three electrons more than the noble gas configuration in the atom so the atom will try to lose three electron to gain noble gas configuration. Therefore, it will form a cation X³⁺.

Formula of its:

(i) Sulphate — X₂(SO₄)₃

(ii) Nitrate — X(NO₃)₃

(iii) Phosphate — XPO₄

(iv) Carbonate — X₂(CO₃)₃

(v) Hydroxide — X(OH)₃

The basic tendency of an atom which makes it to combine with other atoms is the tendency of elements to acquire the nearest noble gas configuration in their outermost orbit and become stable

Compounds formed between metals and non-metals are electrovalent compounds.

The metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have five, six or seven electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent compounds.

(a) Nature of bond is ionic because the atom X gives one electron to each Y atom.

(b) Orbit structure of XY₂ :

(a) Ionic Bond
Reason — Since X has 7 electrons in its outermost shell and Y has only one electron in its outermost shell. So, Y loses its one electron and X gains that electron to form an ionic bond.

(b) The formula of the compound is XY.

(a) Orbit structure of NaCl is shown below:

(b) Orbit structure diagram of calcium oxide is shown below:

(a) Comparison between sodium atom and sodium ion

(b) Comparison between chlorine atom and chloride ion

The electronic configuration of Fluoride ion is the same as that of a neon atom i.e., both have 8 electrons in their valence shell but Fluoride attain that configuration by gaining an electron so it is negatively charged but neon atom have its own 8 valence electrons so it is neutral i.e., it has no charge on it.

(a) Redox Reaction — A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction reaction or redox reaction.

In redox reaction there is a transfer of electrons between two atoms.

(b) The reaction in which electron(s) is gained is called a reduction reaction and the reaction in which there is a loss of electron(s) is called an oxidation reaction

The electronic configuration of:

Potassium (19) = 2, 8, 8, 1

Chlorine (17) = 2, 8, 7

The reaction is :

2K + Cl₂ ⟶ K⁺ + 2Cl^-

(i) Oxidation : 2K ⟶ K⁺
    Potassium undergoes oxidation reaction as it loses an electron.

(ii) Reduction : Cl₂ ⟶ 2Cl^-
     Chlorine undergoes reduction reaction as it gains an electron.

(iii) Chlorine acts as an oxidising agent and gets reduced.

(iv) Potassium acts as a reducing agent and gets oxidised.

A single covalent bond is formed by sharing of one pair of electrons between the atoms, each atom contributing one electron.

Ex: Molecule of hydrogen [H-H]

A double covalent bond is formed by sharing of two pairs of electrons between two atoms.

Ex: Molecule of oxygen [O=O]

A triple covalent bond is formed by sharing of three pairs of electrons between two atoms.

Ex: Molecule of nitrogen [N≡N]

Magnesium loses its two valence electrons to acquire a stable octet. The two chlorine atoms accept one each of these electrons to acquire a stable octet. Thus, electrovalent bond in MgCl₂ is formed by transfer of electrons.
When a molecule of CCl₄ is to be formed, one atom of carbon shares four electron pairs, one with each of the four atoms of chlorine. Thus, covalent bond is formed by sharing of electrons.

(a) Orbital diagram of Magnesium Chloride

(b) Orbital diagram of Nitrogen

(c) Orbital diagram of Methane

(d) Orbital diagram of Hydrogen Chloride

The bond formed between metal (M) and non-metal (chlorine) is ionic bond.

As M bonds with two chlorine atoms having electronic configuration [2,8,7], so we can say that M donates two electrons to two chlorine atoms to complete their octet and therefore, it has a valency of 2.

When metal (M) having +2 valency combines with sulphur [2,8,6] having -2 valency, M donates its two valence electrons to sulphur and formula of compound formed is MgS

When metal(M) having +2 valency combines with oxygen [2,6] having -2 valency, M donates its two valence electrons to oxygen and formula of compound formed is MgO

When metal (M) atoms having +2 valency combines with nitrogen [2,5] having -3 valency then an ionic bond is formed by the transfer of 6 electrons from 3 metal (M) atoms to 2 nitrogen atoms, hence the formula of the compound is Mg₃N₂.

(a) Atom is made of primarily three sub-atomic particles namely electrons, protons and neutrons. An electron has negligible mass, and protons and neutron lie inside the nucleus, hence, mass of an atom is majorly present in the nucleus.

(b) Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas. This is achieved either by gaining electrons or losing electrons or sharing of electrons with other atoms of the same element or another element to form a chemical compound.

(c) Isotopes are the elements having the same atomic number but different mass numbers.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in the number of neutrons.

(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of carbon is same, i.e., electronic configuration of C-12 is [2,4] and electronic configuration of C-14 is [2,4], hence they both have same chemical properties.

(a) Ethene molecule [C₂H₄] — It has one double covalent bond between the 2 Carbon atoms and four single covalent bonds with hydrogen atoms.

(b) Ethyne molecule [C₂H₂] — It has one triple covalent bond between the 2 Carbon atoms and 2 single covalent bonds with hydrogen atoms.

Electrovalent

Reason — Metals become positively charged when they give away electrons and non-metals become negatively charged when they accept electrons. Due to electrostatic attraction between the two oppositely charged ions electrovalent (ionic) bonds are formed.

CsF

Reason — Caesium fluoride CsF, is the most ionic as caesium has the maximum tendency to loose an electron while fluorine has the maximum tendency to gain an electron.

2, 8, 8

Reason — The number of electrons in calcium atom is 20 and the electronic configuration of a neutral calcium atom is 2, 8, 8, 2. Ca²⁺ has two electrons less so the electronic configuration becomes 2, 8, 8.

Redox reaction

Reason — In the formation of an electrovalent bond, the transfer of electron(s) is involved. The electropositive atom loses electrons and undergoes oxidation, while the electronegative atom gains electrons and undergoes reduction. This is known as redox reaction.

Solids

Reason — Ionic compounds have strong electrostatic forces of attraction between their ions, which cannot be separated easily. Thus, they are hard, crystalline solids consisting of ions.

Oxygen molecule

Reason — To attain the stable electronic configuration of the nearest noble gas neon, oxygen needs two electrons.

When two oxygen atoms come closer, each contributes two electrons and so they have two shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a double covalent bond [O=O] between them.

Hydrogen chloride

Reason — The covalent bond is said to be polar when the shared pair of electrons are not at equal distance between the two atoms. This results in the development of fractional positive charge and negative charge on them and they ionise in water. For example: hydrogen chloride.

5

Reason — One carbon atom and 4 chlorine atoms combine to form a carbon tetrachloride (CCl₄) molecule.

Nitrogen

Reason — To attain the stable electronic configuration of the nearest noble gas neon, nitrogen needs three electrons.

When two nitrogen atoms come closer, each contributes three electrons and so they have three shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a triple covalent bond [N≡N] between them.

Neon and argon

Reason —

It is formed between metals and non-metals

Reason — Bonds formed between metals and non-metals are electrovalent bonds.

Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.

It loses electrons and is oxidized.

Reason — As oxidation is loss of electrons so the metal atom is oxidised after losing electrons.

XY₂

Reason — As element X has 2 electrons in its 2nd orbit, so the electronic configuration of X is [2, 2] and Y has 7 electrons in the third orbit so its electronic configuration as [2, 8, 7]

X donates it two valence electrons and completes its octet and two atoms of Y accept the electrons to attain the octet, hence the compound formed is XY₂.

Both Q and R

Reason — An atom which is neutral contains neutrally charged neutrons and positively charged protons [collectively called nucleons] in the central part or nucleus of the atom.

Only P

Reason — According to mordern atomic theory, atoms of an element may not be alike in all respects, as is seen in the case of isotopes.
Modern atomic theory states that atoms can be created and destroyed by nuclear fusion and fission and in the formation of organic compounds, the number of atoms that combine is very big such as thousands. Hence, other two statements are incorrect.

Only R

Reason — An atom as a whole is electrically neutral, because the total positive charge of the nucleus is balanced by the total negative charge of the electrons, i.e., the number of protons and electrons in an atom are equal.

Both Q and R

Reason — Magnesium has two valence electrons in its outermost orbit. So, it will donate its electrons and undergo oxidation to obtain octet configuration. Each chlorine has 7 valence electrons in its outermost orbit. So, it will gain one electron each and undergo reduction to obtain octet configuration.

Both Q and R

Reason — Carbon has the electronic configuration of [2, 4] and Chlorine has 7 electrons in the third orbit so its electronic configuration is [2, 8, 7]. Carbon forms covalent bond by sharing four electron pairs, one with each of the four atoms of chlorine. After sharing of electrons in order to obtain stable octate configuration, carbon attains electronic configuration of neon as [2, 8] and chlorine attains electronic configuration of argon as [2, 8, 8].

Both A and R are true and R is the correct explanation of A.

Explanation — Mass number of an element is the total number of protons and neutrons present in its nucleus. Hence, the asertion (A) is true.
Protons and neutrons each have a mass of about 1 atomic mass unit (amu). Electrons have a mass approximately 1/1836 that of a proton, extremely small and usually neglected in atomic mass calculations. Hence reason (R) is true and reason (R) is the correct explanation of assertion (A).

Both A and R are true but R is not the correct explanation of A.

Explanation — An alpha particle contains two protons and two neutrons, same as helium nucleus. Hence the assertion (A) is true
An alpha particle is formed by removing 2 electrons from helium atom. So, it does not have any electrons. Hence the reason (R) is true.

But reason does not explain why alpha particle is same as helium nucleus. Hence reason (R) is not correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — Isotopes have the same atomic number (Z), i.e., the same number of electrons and protons and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. Thus, isotopes of an element are chemically alike. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — Aluminium (Al) has electronic configuration 2, 8, 3. Al³⁺ is formed when Aluminium loses 3 electrons making its electronic configuration 2, 8. The electronic configuration of neon is also 2, 8. So, Al³⁺ and Ne have the same number of electrons i.e., they are isoelectronic. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — Neon is a noble gas with stable electronic configuration of 2, 8. Hence the assertion (A) is true.
Neon is stable and not reactive because of its electronic configuration which satisfies octet rule, not because of its small size. Hence the reason (R) is false.

A is true but R is false.

Explanation — When an atom changes into an ion, its electrical charge changes, affecting its conductivity, ionic radius, and other physical and chemical characteristics. Hence the assertion (A) is true.
When an atom changes to ion, number of protons and number of neutrons remain same. But number of electrons change. It will either increase or decrease. Hence reason (R) is false.

Both A and R are true but R is not the correct explanation of A.

Explanation — Valency of an element is the combining capacity of that element with other elements and is equal to the number of electrons that take part in a chemical reaction. Hence the assertion (A) is true. The atoms of all elements, other than the inert gases, combine to attain a stable configuration of nearest noble gas. Hence the reason (R) is true.
However, reason (R) is not the correct explanation of assertion (A), because valency is defined by how many electrons are involved in bonding, not by why the atom bonds (which R explains). Hence, while both statements are correct individually, reason (R) doesn’t directly explain assertion (A).

Given,

49.7% of ⁷⁹₃₅Br and

50.3% of ⁸¹₃₅Br

These isotopes are in the ratio = $\dfrac{49.7}{50.3}$ = $\dfrac{497}{503}$

Average atomic mass = $\dfrac{(79 \times 497)+(81 \times 503)}{1000}$

$= \dfrac{39263+(40743)}{1000} \\[1em] = \dfrac{80006}{1000} \\[1em] = 80.006 \text{ amu} \\[1em] \approx 80 \text{ amu}$

∴ Average atomic mass of bromine = 80 amu

Let,

the percentage of ¹⁶₈A be X and

percentage of ¹⁸₈A be 100-X

Average atomic mass = $\dfrac{(16 \times \text{X})+[18 \times \text{(100-X)}]}{100}$ = 16.2

⇒ 16X + 1800 - 18X = 1620

⇒ -2X + 1800 = 1620

⇒ -2X = 1620 - 1800

⇒ 2X = 180

⇒ X = $\dfrac{180}{2}$ = 90

∴ 100 - X = 100 - 90 = 10

Hence, percentage of the two isotopes in this sample are 90% of ¹⁶₈A and 10% of ¹⁸₈A

(a) Covalent Bond — The chemical bond that is formed between two combining atoms by mutual sharing of one or more pair of electrons is called a covalent bond. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.

(b) (i) Bond formed between two H atoms; H-H

A hydrogen atom has one electron in its only shell. It needs one more electron to attain duplet. To meet this need, hydrogen atom shares its electron with another hydrogen atom. Thus, one electron each is contributed by the two atoms of hydrogen, and the resulting pair of electrons is mutually shared by both the atoms to form a hydrogen molecule.

(ii) Bond formed between a hydrogen atom and chlorine atom; H‒Cl

In the case of hydrogen chloride molecule, hydrogen atom shares an electron pair with chlorine atom, such that hydrogen acquires a duplet configuration and chlorine an octet, resulting in the formation of a single covalent bond.

Element A with 1 electron in its first shell is hydrogen, and element B with 7 electrons in its third shell is chlorine. So, a single covalent bond is formed between hydrogen and chlorine by sharing one pair of electron.

(i) Y will have negative charge as electrons are getting added to it.

(ii) Y is getting reduced as reduction is gain of electrons.

(a) X[6] = Electronic configuration [2,4]

Y[9] = Electronic configuration [2,7]

Z[12] = Electronic configuration [2,8,2]

(i) Y gains one electron to attain a stable octet, hence becomes negatively charged anion.

(ii) Z loses two electrons to attain a stable octet hence becomes positively charged cation.

(b) Ionic bond with molecular formula ZY₂.

Potassium chloride being an electrovalent compound consists of ions. In aqueous solution state, the electrostatic forces between them weaken and the ions become mobile conducting electricity.

On the other hand, hydrogen chloride is a polar covalent compound. The shared electron pair is shifted towards Chlorine thereby developing a slight negative charge on it and a slight positive charge on hydrogen atom. In its aqueous solution state, hydrogen chloride ionises into Hydronium and Chloride ions as shown in the equation below. These ions help in conducting electricity.

HCl + H₂O ⇌ H₃O⁺ + Cl^-

Hydrogen Chloride (HCl) & Ammonia (NH₃)

With Chlorine — MCl₂

With Sulphur — MS

Explanation:

Given, M reacts with oxygen to form an ionic bond MO. Oxygen needs 2 electrons to complete its octet forming the anion O^2-. As the compound formed is MO, it means that M has 2 electrons in its valence shell which it donates to Oxygen to attain a stable electronic configuration forming a cation M²⁺.

Each Chlorine atom accepts 1 electron to complete its octet so 2 Chlorine atoms combine with 1 atom of M to form MCl₂. Each Sulphur atom accepts 2 electrons to complete its Octet so 1 Sulphur atom combines with 1 atom of M to form MS.

Covalent bonding
Reason — The valency of hydrogen element is 1 and that of oxygen is 2. Hydrogen needs one electrons to attain stable duplet structure of nearest noble gas - He [2] and oxygen needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
Each of the two hydrogen atoms shares an electron pair with the oxygen atom such that hydrogen acquires a duplet configuration and oxygen an octet configuration resulting in the formation of two single covalent bonds [H-O-H] in the molecule of water.

(ii) Electrovalent bonding
Reason — The valency of calcium (2,8,8,2) element and that of oxygen (2,6) is 2. Calcium donates two electrons and oxygen takes up those two electrons and electrovalent bond is formed.

(iii) Polar covalent bonding Reason — In hydrogen chloride, the strong nuclear charge of the chlorine atom attracts the shared electron pair towards itself, i.e., negative charge shifts towards the chlorine atom thereby developing a slight negative charge on it. The hydrogen atom develops a slight positive charge. Therefore, a polar covalent bond is formed as shown below :

(a) (i) A and B are isotopes, as they have same atomic number but different mass number.

(ii) X and Y are isobars, as they have same mass number but different atomic number.

(b) (i) Examples of isotopes — ³⁵₁₇Cl, ³⁷₁₇Cl

(ii) Examples of isobars — ⁴⁰₁₈Ar, ⁴⁰₂₀Ca

(a) [2, 8, 6], as it has three shells — K, L, M.

(b) 16, as atomic number is the total number of electrons so, 2+8+6 = 16

(c) Non-metal, as it has 6 electrons in the valence shell and will accept 2 electrons to complete its octet.

(d) Anion, A^2-, as element A has 6 valence electrons so it has a valency of -2 and it will take two electrons, complete its octet and become a negatively charged ion.

(e) Sulphur, as sulphur also has the electronic configuration of [2, 8, 6].

(f) Orbital structure of A and its ion is shown below:

(a) Mass number is indicated by figure 9, which is the sum of protons and neutrons in the nucleus of an atom.

(b) Atomic number is indicated by figure 4, which is the number of electrons present in a neutral atom.

(c) 4, as the atomic number also indicates the number of protons in the nucleus of an atom.

(d) 5, number of neutron = mass number - number of protons = 9-4=5

(e) 4, as the atomic number is 4, which is the number of electrons present in a neutral atom.

(f) 2, its electronic configuration is [2, 2]

(g) Y²⁺, as it has the valency of +2, hence it loses two valence electrons and becomes a positively charged ion.

(a)

(b) A and C have different number of nucleon, because the number of protons is same in both but number of neutrons are different.

(c) Isotopes

(d) Isotopes are atoms of the same element having the same atomic number but different mass numbers.

(a) Cation [positively charged] and anion [negatively charged]

(b) In the formation of covalent compounds, the electrons are shared between the combining atoms. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.

(c) Two electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule.
Reason — Nitrogen has 5 electrons in the valence shell whereas it forms triple bond to fulfill it's octet and hence, 2 electrons are left in the outer shell of a nitrogen atom which are not involved in the formation of a nitrogen molecule.

(d) Magnesium is oxidized and Chlorine is reduced.
Reason — In the formation of magnesium chloride the reaction is :

Mg + Cl₂ ⟶ Mg²⁺ + 2Cl^-

Oxidation : Mg ⟶ Mg²⁺

Reduction : Cl₂ ⟶ 2Cl^-

(a) XY₂
Reason — As element X has 2 electrons in its M shell, so the electronic configuration of X is [2, 8, 2] and Y has 7 electrons in the third shell so its electronic configuration is [2, 8, 7]
X donates it two valence electrons and completes its octet and two atom of Y accepts the electrons to attain the octet, hence the compound formed is XY₂.

(b) The nearest inert gas electronic configuration for element X is 2, 8 i.e., Neon while that for element Y is 2, 8, 8 i.e, Argon.

(c) Orbital diagram of XY₂

(i) In the formation of Oxygen molecule:

(a) Neon (10) 2,8, because electronic configuration of oxygen is [2,6] so to attain the stable configuration of nearest noble gas Neon, oxygen needs two electrons.

(b) Two pairs of electrons are shared, and both oxygen atoms attain their octet.

(c) Double Covalent bond, as the bond between two oxygen atoms is formed by sharing of electrons.

(d) Orbital Diagram of the formation of Oxygen molecule:

(ii) In the formation of Carbon tetra chloride molecule:

(b) Four pair of electrons are shared, as carbon needs four electrons and chlorine needs one electron to complete their octet, hence four chlorine atoms share their one electron each with carbon and all the atoms attain a stable configuration.

(c) Single Covalent bond, as the bond between carbon and 4 chlorine atoms is formed by sharing one pair of electrons.

(d) Orbital Diagram of the formation of Carbon tetrachloride molecule:

Explanation:

1. Electronic configuration of an element with atomic number 15 is [2, 8, 5], hence, it needs three electrons to complete its octet and forms a trivalent ion.
2. Electronic configuration of an element with atomic number 19 is [2, 8, 8, 1], hence, it has K, L, M, N shells.
3. Electronic configuration of an element with atomic number 8 is [2, 6], hence it has 6 electrons in the outermost shell called valence electrons.
4. Electronic configuration of an element with atomic number 10 is [2, 8], so its octet is complete and it does not form ions.

(i) Reduction, since there is a gain of 2 electrons by Oxygen.

(ii) Oxidation, since there is a loss of an electron by Potassium.

(iii) Reduction, since there is a gain of an electron by Iron.

(iv) Oxidation, since there is a loss of two electrons by Zinc.

(a) Single Covalent bond

(b) Electrovalent bond

(a) Helium (Inert element)

(b) Lithium

(c) Isotopes

(d) Isobars

(e) Electrovalent or Ionic bond

(f) Anion