Chemical Bonding

NCERT Solution

Part 2

Question 12: H₃PO₃ can be represented by structures 1 and 2 shown below. Can these two structures be taken as the canonical forms of the resonance hybrid representing H₃PO₃? If not, give reasons for the same.

Answer: As position of atoms has changed in these structures so these cannot be taken as the canonical forms of resonance hybrid of this molecule.

Question 13: Write the resonance structures for SO₃, NO₂ and NO₃^-.

Answer:

Question 14: Use Lewis symbols to show electron transfer between the following atoms to form cations and anions: (a) K and S (b) Ca and O (c) Al and N.

Answer: K and S combine to for K₂S

Ca and O combine to form CaO

Al and N combine to form AlN

Question 15: Although both CO₂ and H₂O are triatomic molecules, the shape of H₂O molecule is bent while that of CO₂ is linear. Explain this on the basis of dipole moment.

Answer: Net dipole moment of CO₂ is zero, while it is 1.84 in case of H₂. Due to this, CO₂ molecule is linear but H₂ molecule is bent.

Question 16: Write the significance/applications of dipole moment.

Answer: Dipole moment helps in predicting the nature of the molecule. A molecule with zero dipole moment is non-polar, while a molecule with dipole moment is polar.

Question 17: Define electronegativity. How does it differ from electron gain enthalpy?

Answer: Ability of an atom to attract bond pair of electrons is called electronegativity. For a given atom, electronegativity is not constant but depends on the atom to which it combines. Electronegativity is a relative quantity and cannot be measured. On the other hand, electron gain enthalpy is the change in energy when an atom gains electron. Electron gain enthalpy is measurable and is constant for a given atom. Electron gain enthalpy can be negative or positive; depending on gain or loss of electron.

Question 18: Explain with the help of suitable example polar covalent bond.

Answer: In case of a heteronuclear molecule, the shared pair of electrons gets displaced more towards the more electronegative atom. The bond so formed is called polar covalent bond. The electronegative atom becomes slightly negatively charged, and electropositive atom acquires slight positive charge. For example; in case of HF, the shared pair of electrons is displaced towards fluorine.

Question 19: Arrange the bonds in order of increasing ionic character in the molecules: LiF, K₂O, N₂, SO₂ an ClF₃.

Answer: N₂ < SO₂ < ClF₃ < K₂O < LiF

Question 20: The skeletal structure of CH₃COOH as shown below is correct, but some of the bonds are shown incorrectly. Write the correct Lewis structure for acetic acid.

Answer:

Question 21: Apart from tetrahedral geometry, another possible geometry for CH₄ is square planar with the four H atoms at the corners of the square and the C atom at its centre. Explain why CH₄ is not square planar?

Answer: Electronic configuration of Carbon in ground state: [He] 2s²2p²

Electrons configuration of C in excited state: [H] 2s¹ 2p_x 2p_y 2p_z

Now, the four atomic orbitals of carbon can overlap with 1s orbitals of four H atoms. This will result in formation of four C-H bonds. While the three p orbitals of C are at 90° to one another, the HCH angle for these will also be 90°. This means that three C-H bonds will be oriented at 90° to one another. The 2s orbital of C and 1s orbital of H are spherically symmetrical and so they can overlap in any direction. Hence, the direction of fourth C-H bond cannot be ascertained. But this description does not fit with the tetrahedral HCH angles of 109.5°. This shows why CH₄ is tetrahedral and not square planar.

Question 22: Explain why BeH₂ molecule has zero dipole moment although the Be-H bonds are polar.

Answer: Two equal bonds are opposite each other and their dipole moments cancel each other. Due to this, BeH₂ has zero dipole moment.

Question 23: Which out of NH₃ and NF₃ has higher dipole moment and why?

Answer: Dipole moment of NH₃ (4.90 × 10^-30 C m) is greater than that of NF₃ (0.8 × 10^-30 C m). This happens because in case of NH₃ the orbital dipole due to lone pair is in same direction as the resultant dipole. But in case of NF₃ the orbital dipole is in opposite direction to the resultant dipole.

Question 24: What is meant by hybridization of atomic orbitals? Describe the shapes of sp, sp², sp³ hybrid orbitals.

Answer: The process of intermixing of orbitals of slightly different energies resulting in the formation of new set of orbitals of equivalent energies and shape is called hybridization. The concept of hybridization was proposed by Pauling and we can explain the shapes of polyatomic molecules on this concept. For example; when one 2s and three 2p orbitals of carbon hybridise, four new sp³ hybrid orbitals are formed.

sp Hybridization: A molecule in which the central atom is sp-hybridized and is directly linked to two other atoms, the central atom possesses linear geometry.

sp² Hybridization: It involves merger of one s and two p orbitals resulting in the formation of three equivalent sp² hybridized orbitals. Molecule of BCL₃ gives an example of sp² hybridization. The hybridized orbitals which are oriented in a trigonal planar arrangement and overlap with 2p orbitals of chlorine to form three B-Cl bonds.

sp³ Hybridization: It involves merger of one s and three p orbitals. The four sp³ orbitals thus formed are directed towards the four corners of a tetrahedron at an angle of 109.5°. CH₄ gives a good example of this type of hybridization.