Structure of the Atom

Limitations of J.J. Thomson's model of the atom were:

1. It failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain the stability of an atom.
2. The concept of the nucleus was not present in his model.
3. It failed to explain the results of Rutherford's ∝–particle scattering experiment.

1. The revolution of the electron in a circular orbit is not expected to be stable.
2. Any particle in circular orbit would undergo acceleration, and hence, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus.
3. Hence, atoms must be highly unstable and matter would not exist in its known form. We know that atoms are quite stable.

Neil Bohr put forward the following postulates about the model of an atom:

1. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
2. While revolving in discrete orbits the electrons do not radiate energy.

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:

1. The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the shell number.
   K shell, n=1, no. of electrons = 2 x 1² = 2
   L shell, n=2, no. of electrons = 2 x 2² = 8
   M shell, n=3, no. of electrons = 2 x 3² = 18
2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
3. The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

The definite combining capacity of the atoms of each element, wherein electrons are lost, gained or shared to make the octet of electrons present in the outermost shell, is defined as valency.

(i) The electronic configuration of silicon [14] is 2, 8, 4. It has 4 electrons in its outermost shell. So, the valency of silicon = 8 – the number of electrons in the outermost shell = 8 - 4 = 4.

Hence, valency of silicon = 4.

(ii) The electronic configuration of oxygen [8] is 2, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of oxygen = 8 – the number of electrons in the outermost shell = 8 − 6 = 2.

Hence, valency of oxygen = 2.

(i) Atomic number — The number of protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z.

Example: Atomic number of hydrogen (Z) = 1 as only one proton is present in the nucleus of hydrogen atom.

(ii) Mass number — The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and is denoted by A.

Example: Mass number of Carbon (A) = 12 as it has 6 protons and 6 neutrons.

(iii) Isotopes — The atoms of the same element having the same atomic number but different mass number are known as isotopes.

Example: Carbon has two isotopes ₆C¹² and ₆C¹⁴

(iv) Isobars — Atoms of different elements with different atomic numbers, which have same mass number are known as isobars.

Example: Calcium [₂₀Ca⁴⁰] atomic number 20 and argon [₁₈Ar⁴⁰] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.

Isotopes have special properties which makes them useful in various fields like:

1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.

The atomic number of sodium is 11. It has 11 electrons in its orbitals. So, its electronic configuration is K-2, L-8, M-1. The maximum electrons in K, L, M shells according to 2n² rule are 2, 8 and 18 respectively.

In case of Na⁺ (10), one electron from the M shell is lost, and it obtains a positive charge. The new electronic configuration is K-1 and L-8. Hence, it has completely filled shells.

The atomic mass of an element is the mass of one atom of that element.

Average atomic mass takes into account the isotopic abundance.

Isotope of bromine with atomic mass 79 u = 49.7%

Therefore, Contribution of ₃₅⁷⁹Br to atomic mass = $\dfrac{79 \times 49.7}{100}$ = 39.263 u

Isotope of bromine with atomic mass 81 u = 50.3%

Contribution of ₃₅⁸¹Br to the atomic mass of bromine = $\dfrac{81 \times 50.3}{100}$ = 40.743u

Hence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u

Let the percentage of ₈¹⁶X be a and that of ₈¹⁸X be 100 - a.

16.2u = $\dfrac{\text{16a}}{100}$ + $18\dfrac{\text{(100-a)}}{100}$

⇒ 1620 = 16a + 1800 – 18a

⇒ 1620 = 1800 – 2a

⇒ 2a = 1800 - 1620

⇒ a = $\dfrac{1800 - 1620}{2}$

⇒ a = $\dfrac{180}{2}$

⇒ a = 90

∴ Percentage of ₈¹⁶X = 90%

and

Percentage of ₈¹⁸X = 100 - a = 100 - 90 = 10%

Hence, the percentage of the isotope ₈¹⁶X in the sample is 90% and that of ₈¹⁸X is 10%

Given,

Atomic number, Z = 3

The electronic configuration of the element = 2,1. Hence, its valency = 1

The element with atomic number 3 is Lithium.

Mass number = No. of protons + No. of neutrons

Mass number of X = 6 + 6 = 12

Mass number of Y = 6 + 8 = 14

Number of protons is same in both so atomic number is same however as the number of neutrons is different so mass number is different. Hence, these two elements are isotopes.

(a) False.

Reason — Thomson proposed that:

1. An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

(b) False.

Reason — Neutron is not formed by an electron and a proton combining together. It is present in the nucleus of an atom alongside protons.

(c) True.

Reason — The mass of an electron is about $\dfrac{1}{2000}$ times that of a proton.

(d) False.

Reason — Tincture iodine is an antiseptic that usually contains 2-7% elemental I-127 dissolved in a mixture of ethanol and water. Normal iodine is used in its preparation, not its isotope. ​

Atomic nucleus

Reason — Rutherford's alpha–particle scattering experiment was responsible for the discovery of atomic nucleus.

Different number of neutrons

Reason — Isotopes of an element have same atomic number (i.e., number of protons) and different mass number (i.e., sum of protons + neutrons).

8

Reason — The electronic distribution of Cl is 2, 8, 7. There are 7 valence electrons in chlorine. It gains one electron and forms Cl^-. Hence, its valency is 8.

2, 8, 1

Reason — Sodium has atomic number 11 and it's electronic configuration is 2,8,1.

The canal rays are positively charged radiations present in a gas discharge. They helped in the discovery of another sub-atomic particle that was positively charged called the proton.

As the atom contains one electron, so it is negatively charged whereas the presence of proton makes the atom positively charged particle. The net charge becomes zero as both particles neutralize the charge present on each other.

As per Thompson's model of an atom,

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) Electrons (negatively charged) and protons (positively charged) are equal in magnitude. So, the atom as a whole, is electrically neutral.

As per Rutherford's model of an atom, the positively charged protons are present in the nucleus of an atom.

Below diagram shows Bohr's model of an atom with three shells:

If the ∝–particle scattering experiment is carried out using a foil of a metal (as thin as gold) other than gold, then also the observation would be same because the structure of an atom, when considered individually is same.

However, other metals are not as malleable as gold so it is difficult to turn those metals into such thin sheets. If thick foil is used then it will result in the change of scattering angles which might affect the predictions of the experiment.

The three subatomic particles of an atom are:

1. Protons – Positively charged
2. Electrons – Negatively charged
3. Neutrons – No charge (neutral)

Given,

Atomic mass of helium atom = 4u,

2 protons in helium nucleus

We know,

Atomic mass = number of protons + number of neutrons

4 = 2 + number of neutrons

So, number of neutrons = 4 – 2 = 2

Hence, Helium has 2 neutrons.

(i) A carbon atom has 6 electrons.

In the first orbit or K-shell = 2 electrons;

In the second orbit or L-shell = 4 electrons;

We can also express the electron distribution in a carbon atom as 2, 4.

(ii) A sodium atom has 11 electrons.

In the first orbit or K shell = 2 electrons;

In the second orbit or L-shell = 8 electrons;

In the third orbit or M-shell = 1 electron

We can express the electron distribution in a sodium atom as 2, 8, and 1.

K shell can hold 2 electrons and L shell can hold 8 electrons according to 2n² rule . When both the shells are full, the total number of electrons present in the atom = 2 + 8 = 10 electrons.

(i) Valency of Chlorine :

The electronic configuration of chlorine = 2, 8, 7. It has 7 (more than 4) electrons in its outermost shell. So, the valency of chlorine = 8 – the number of electrons in the outermost shell = 8 - 7 = 1.

Hence, the valency of Cl is 1.

(ii) Valency of Sulphur:

The electronic configuration of Sulphur = 2, 8, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of S = 8 – the number of electrons in the outermost shell = 8 - 6 = 2.

Hence, the valency of S is 2.

(iii) Valency of Magnesium:

The electronic configuration of Magnesium = 2, 8, 2. It has 2 (less than 4) electrons in its outermost shell. So, the valency of magnesium = Number of electrons in its outermost shell = 2

Hence, the valency of Mg is 2.

Given,

Number of electrons = 8

Number of protons = 8

(i) As the atomic number of an atom is the number of electrons in that atom; hence, atomic number = 8.

(ii) As the number of electrons and protons are equal in number. Hence, both the charges (positive and negative) neutralize each other and the atom does not possess any charge.

(a) Mass number of oxygen = ?

Given,

Number of protons = 8

Number of neutrons = 8

Mass number = Number of protons + number of neutrons = 8 + 8 = 16

∴ Mass number of oxygen = 16

(b) Mass number of Sulphur = ?

Number of protons = 16

Number of neutrons = 16

Mass number = Number of protons + number of neutrons = 16 + 16 = 32

∴ Mass number of sulphur = 32

(a) Isotopes — Carbon has two isotopes i.e., ₆C¹² and ₆C¹⁴. They have same electronic configuration as they have same atomic numbers = 6. Hence, their electronic configuration is 2, 4.

(b) Isobars — Calcium [₂₀Ca⁴⁰] atomic number 20 and argon [₁₈Ar⁴⁰] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.

Electronic configuration of ₂₀Ca⁴⁰ is 2, 8, 8, 2.

Electronic configuration of ₁₈Ar⁴⁰ is 2, 8, 8.