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Solutions for Chemistry, Class 9, ICSE
Explain the term 'chemical bond' and 'chemical bonding'.
Answer
The force which acts between two or more atoms to hold them together as a stable molecule is called a chemical bond and the concept is called chemical bonding.
State why noble gases have stable electronic configuration while atoms of other elements have unstable electronic configuration.
Answer
Octet and duplet structures seem to represent stability and are said to be stable electronic configuration.
Noble gases have their outermost or valence shell complete i.e., they have 2 electrons in outermost shell e.g., helium [electronic configuration 2] or they have 8 electrons in outermost shell e.g., neon [electronic configuration 2,8] or argon [electronic configuration 2,8,8 ]. Hence, they have stable electronic configuration. They do not gain, lose or share electrons and are unreactive or inert.
Atoms of other elements have incomplete valence shell, hence they have unstable electronic configuration. So, they can gain, lose or share electrons to attain the stable electronic configuration of the nearest noble gas.
Explain in brief how atoms of other elements other than noble gases, attain stable electronic configuration of the nearest noble gas.
Answer
Atoms of elements other than noble gases, attain stable electronic configuration of the nearest noble gas by:
Define each of the following terms -
(a) electrovalent bond
(b) electrovalent compound
(c) covalent bond
(d) covalent compound
with suitable examples wherever required.
Answer
(a) The chemical bond formed between the two atoms by transfer of one or more electrons from the atom of a metallic element to an atom of a non-metallic element is called an electrovalent bond.
(b) The chemical compound formed as a result of transfer of one or more electrons from the atom of a metallic element to an atom of a non-metallic element is called an electrovalent compound. e.g., NaCl, CaO
(c) The chemical bond formed due to mutual sharing of electrons between the given pairs of atoms of non-metallic elements is called a covalent bond.
(d) The chemical compound formed due to mutual sharing of electrons between the given pair of non-metallic elements is called a covalent compound. e.g., Chlorine molecule, Oxygen molecule.
State what is meant by - (a) duplet rule (b) octet rule.
Answer
(a) Duplet Rule — For an atom to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium.
(b) Octet Rule — For an atom to achieve stable electronic configuration it must have 8 electrons in outermost orbit like that of noble gases other than Helium.
Sodium chloride is formed as a result of - transfer of electrons from metallic sodium atom to non-metallic chlorine atom. With the help of atomic orbit structure diagram - show the formation of sodium chloride [Atomic numbers: Na = 11, Cl = 17].
Answer
Atomic orbit structure diagram of the formation of sodium chloride by transfer of electrons from metallic sodium atom to non-metallic chlorine atom is shown below:
Atomic number of calcium is 20 and of oxygen is 8. State the number of electrons - calcium loses and oxygen gains to attain stable electronic configuration of the nearest noble gas - during formation of electrovalent molecule, calcium oxide.
Answer
Calcium oxide is formed as a result of transfer of two valence electrons
Explain with the help of atomic orbit structure diagram the formation of calcium oxide.
Answer
Atomic orbit structure diagram of the formation of calcium oxide is shown below:
Calcium oxide is formed as a result of transfer of two valence electrons
Give a reason why - in the formation of electrovalent compound - magnesium chloride, one magnesium atom combines with two chlorine atoms to give magnesium chloride. [at. no. Mg=12, Cl=17, ]
Answer
Magnesium [At. no. 12 and electronic configuration 2,8,2] loses two electrons from it's outer shell in order to attain stability of nearest noble gas Neon [2,8] and becomes a magnesium ion [cation] : Mg2+.
Chlorine [At. no. 17 and electronic configuration 2,8,7] accepts one electron in it's outer shell to attain stability of nearest noble gas Argon [2,8,8] and becomes a chloride ion [anion] : Cl1-.
Magnesium atom donates two electrons, whereas chlorine accepts one electron to give - chloride ion. Therefore, to accept the other electron donated by magnesium atom there must be another chlorine atom. Thus, one magnesium atom combines with two chlorine atoms to give MgCl2
'Formation of hydrogen molecule takes place by sharing of electrons'. Give a reason why the molecule of hydrogen is not formed by transfer of electrons.
Answer
Hydrogen is a non-metal and has 1 valence electron, so in order to attain a stable duplet electronic configuration, it tries to gain one more electron. As both the combining atoms are one electron short, therefore, none of the two atoms are in a position to donate. Hence, they share 1 valence electron and get a stable duplet configuration.
Draw the atomic orbit structure diagram for formation of -
(a) hydrogen molecule
(b) chlorine molecule
(c) oxygen molecule
(d) nitrogen molecule
[at. no. H=1, Cl=17, O=8, N=7]
Answer
(a) Atomic orbit structure diagram for formation of hydrogen molecule is shown below:
(b) Atomic orbit structure diagram for formation of chlorine molecule is shown below:
(c) Atomic orbit structure diagram for formation of oxygen molecule is shown below:
(d) Atomic orbit structure diagram for formation of nitrogen molecule is shown below:
Give reasons for formation of single covalent bond between two chlorine atoms and a double covalent bond between two oxygen atoms - during formation of a covalent chlorine molecule and formation of a covalent oxygen molecule, respectively.
Answer
Atomic number of Cl is 17 and elect. config. is [2,8,7]. As both the Cl atoms require one electron to complete it's octet, hence, they both will share 1 electron and will attain a stable electronic configuration of the nearest noble gas — argon. Therefore, single covalent bond is formed during formation of a chlorine molecule.
Atomic number of O is 8 and elect. config. is [2,6]. As both the O atoms require two electrons to complete their octet, hence, they both will share 2 electrons and will attain a stable electronic configuration of the nearest noble gas — neon. Therefore, double covalent bond is formed during formation of a oxygen molecule.
Draw the atomic orbit structure diagram for formation of a -
(a) water molecule
(b) ammonia molecule
[at. no. H=1, O=8, N=7]
Answer
(a) Atomic orbit structure diagram for formation of water molecule is shown below:
(b) Atomic orbit structure diagram for formation of ammonia molecule is shown below:
State why water has two lone pairs of electrons in it's covalent molecule while ammonia has one lone pair.
Answer
Water contains two hydrogen atoms and one oxygen atom. The hydrogen atom [1] has one electron in it's valence shell while the oxygen atom [2,6] has 6 electrons in it's valence shell. Two hydrogen atom share one electron each and two unshared pairs of electrons are left as lone pairs as shown in the diagram below :
In the case of ammonia, it contains three hydrogen atoms and one nitrogen atom. Nitrogen atom [2,5] has five valence electrons and hydrogen atom [1] has one valence electron. In ammonia, each of the three hydrogen atoms are bonded to the nitrogen atom by a pair of shared electrons. Thus, the nitrogen atom in ammonia is left with one lone pair of electrons as shown in the diagram below:
With the help of an atomic orbit structure diagram - explain the formation of :
(a) carbon tetrachloride
(b) methane
[at. no. : C=6, Cl=17, H=1]
Answer
(a) Atomic orbit structure diagram for formation of carbon tetrachloride molecule is shown below:
(b) Atomic orbit structure diagram for formation of methane molecule is shown below:
Give a reason why one atom of carbon shares four electron pairs, one with each of the four atoms of chlorine during the formation of covalent molecule - carbon tetrachloride.
Answer
Carbon [At. no. 6, electronic configuration 2,4] needs four electrons to attain the electronic configuration of nearest noble gas — Neon. Chlorine atom [At. no. 17, electronic configuration 2,8,7] needs one electron to attain stable electronic configuration of nearest noble gas — Argon. Hence, to attain octet one atom of carbon shares four electron pairs, one with each of the four atoms of chlorine and forms covalent molecule carbon tetrachloride.
Compound X consists of molecule.
(i) Bonding in X will be:
A: ionic
B: electrovalent
C: covalent
D: molecular
Answer
Covalent
Reason — As compound X consists of molecules, it means that compound X is a Covalent compound (electrovalent compounds consists of ions) hence bonding in X is covalent.
What is a lone pair of electrons.
Answer
Lone pair of electrons are valence electrons that in a covalent bond are not shared with another atom.
The electronic configuration of nitrogen is 2,5. How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule.
Answer
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. Formation of a Nitrogen molecule from Nitrogen atoms is shown in the below diagram:
What is the term defined in:- a bond formed by shared pair of electrons each bonding atom contributing one electron to the pair.
Answer
Single covalent bond
Draw the atomic orbit structure of carbon tetrachloride and state the type of bond present in it.
Answer
Atomic orbit structure of carbon tetrachloride is shown below:
Type of bond present in carbon tetrachloride is Covalent Bond
Select the right answer -
(i) Sodium chloride — covalent bond/ionic bond/covalent and coordinate bond.
(ii) Carbon tetrachloride — covalent bond/ionic bond/covalent and coordinate bond.
Answer
(i) Sodium chloride — ionic bond.
(ii) Carbon tetrachloride — covalent bond.
(i) In covalent compounds, the bond is formed due to ............... [sharing/transfer] of electrons.
(ii) A molecule of ............... contains a triple bond. [hydrogen/ammonia/nitrogen]
Answer
(i) In covalent compounds, the bond is formed due to sharing of electrons.
(ii) A molecule of nitrogen contains a triple bond.
The molecule which contains a triple covalent bond is:
A: ammonia
B: methane
C: water
D: nitrogen
Answer
Nitrogen
Reason — As the electronic configuration of N is [2,5] so when two nitrogen atoms come closer, each contributes three electrons and they have three shared pair of electrons between them. Therefore, it forms triple covalent bond [N ≡ N].
Explain the bonding in methane molecule.
Answer
Methane consists of one carbon atom [2,4] and four hydrogen atoms [1]. Carbon needs four electrons to attain a stable electronic configuration of nearest noble gas - Ne, whereas, hydrogen needs one electron to attain stable electronic configuration of nearest noble gas - He.
Therefore, one atom of carbon shares four electron pairs one with each of the four hydrogen atoms, resulting in formation of methane molecule.
The atomic orbit structure diagram of the formation of methane molecule is shown below:
Name the following:
Answer
Give reasons for the following :
To attain stable electronic configuration of the nearest noble gas —
Answer
Complete the table given below, pertaining to formation of covalent compounds :
| Formation of a covalent compound between | Atomic number | No. of covalent bonds in molecule |
|---|---|---|
| (a) Two atoms of 'A' | 9 | |
| (b) Two atoms of 'B | 7 | |
| (c) Two atoms of 'C' | 8 | |
| (d) Two atoms of 'D' | 1 | |
| (e) Two atoms of 'E' | 17 |
Answer
| Formation of a covalent compound between | Atomic number | No. of covalent bonds in molecule |
|---|---|---|
| (a) Two atoms of 'A' | 9 | 1 covalent bond |
| (b) Two atoms of 'B | 7 | 3 covalent bonds |
| (c) Two atoms of 'C' | 8 | 2 covalent bonds |
| (d) Two atoms of 'D' | 1 | 1 covalent bond |
| (e) Two atoms of 'E' | 17 | 1 covalent bond |
Differentiate between the following with a suitable example :
Lone pair & shared pair of electrons.
Answer
| Lone Pair | Shared Pair |
|---|---|
| Lone pair of electrons are valence electrons that in a covalent bond are not exchanged with another atom and is often called an unshared pair or non-bonding pair. | Shared pair of electrons are valence electrons shared by two atoms to form covalent bond. |
| Example: Ammonia molecule has one lone pairs of electrons | Example: Water has two shared pair of electrons. |
Differentiate between the following with a suitable example :
Duplet rule & octet rule.
Answer
| Duplet Rule | Octet Rule |
|---|---|
| For an atom to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium. | For an atom to achieve stable electronic configuration it must have 8 electrons in outermost shell like that of noble gas other than Helium. |
Differentiate between the following with a suitable example :
Stable electronic configuration & unstable configuration.
Answer
| Stable Electronic Configuration | Unstable Electronic Configuration |
|---|---|
| Atoms having their valence shell completely filled are said to have Stable Electronic Configuration. | Atoms which do not have their valence shells completely filled are said to have Unstable Electronic Configuration. |
| Atoms of noble gases have Stable Electronic Configuration. | Atoms of elements other than noble gases have Unstable Electronic Configuration. |
| In atoms with Stable Electronic Configuration, electrons are arranged as per Duplet or Octet rule. | In atoms with Unstable Electronic Configuration, arrangement of electrons does not follow Duplet or Octet rule. |
Differentiate between the following with a suitable example :
Electrovalent bond & covalent bond
Answer
| Electrovalent bond | Covalent bond |
|---|---|
| The chemical bond formed between the two atoms by transfer of one or more electrons from the atom of a metallic element to an atom of a non-metallic element is called an electrovalent bond. | The chemical bond formed due to mutual sharing of electrons between the given pair of atoms of non-metallic elements is called a covalent bond. |
| e.g., NaCl, CaO | e.g., Chlorine molecule, oxygen molecule |
Differentiate between the following with a suitable example :
Single covalent bond & triple covalent bond.
Answer
| Single Covalent Bond | Triple Covalent Bond |
|---|---|
| The chemical bond formed due to mutual sharing of one pair of electrons between the given pair of atoms of non-metallic elements is called a Single Covalent Bond. | The chemical bond formed due to mutual sharing of three pairs of electrons between the given pair of atoms of non-metallic elements is called a Triple Covalent Bond. |
| E.g., Hydrogen [H - H] | E.g., Nitrogen [N ≡ N] |
Atomic numbers of the following elements are given below:
A = 8; B = 7; C = 17; D = 11; E = 20
Answer
The representation below shows the outline formation of an electrovalent compound.
If the atomic number of element 'X' is 11 and of element 'Y' is 17 —
Answer