VBT

Question 1

What does the Lewis approach fail to explain?

Answer 1

Formation of chemical bonds, differences in bond dissociation enthalpies and bond lengths, shapes of polyatomic molecules.

Question 2

What theory gives the geometry of simple molecules but has limited applications?

Answer 2

SEPR theory.

Question 3

What are the two important theories based on quantum mechanical principles introduced to overcome the limitations of Lewis and SEPR theories?

Answer 3

Valence bond (VB) theory and molecular orbital (MO) theory.

Question 4

Who introduced valence bond theory?

Answer 4

Heitler and London in 1927.

Question 5

Which scientists further developed valence bond theory?

Answer 5

Pauling and others.

Question 6

What key aspects is valence bond theory based on?

Answer 6

• Atomic orbitals
• Electronic configurations
• Overlap criteria of atomic orbitals
• Hybridization of atomic orbitals
• Principles of variation and superposition.

Question 7

What is the simplest molecule discussed in valence bond theory?

Answer 7

Hydrogen molecule.

Question 8

What happens to two hydrogen atoms as they approach each other?

Answer 8

New attractive and repulsive forces begin to operate.

Question 9

What attractive forces arise between hydrogen atoms?

Answer 9

Nucleus of one atom and its own electron (Na-e and Nb-e).

Question 10

Fill in the blank: Valence bond theory has been discussed in terms of _______.

Answer 10

Qualitative and non-mathematical treatment.

Question 11

True or False: Valence bond theory provides a rigorous mathematical treatment of atomic orbitals.

Answer 11

False.

Question 12

What is the bond dissociation enthalpy of H2?

Answer 12

435.8 kJ/mol.

Question 13

What is the bond length of F2?

Answer 13

144 pm.

Question 14

What is the bond dissociation enthalpy of F2?

Answer 14

155 kJ/mol.

Question 15

What does bond length refer to?

Answer 15

The distance between the nuclei of two bonded atoms.

Question 17

What forces arise between the nucleus of one atom and the electrons of another atom?

Answer 17

Attractive and repulsive forces

Attractive forces pull atoms together, while repulsive forces push them apart.

Question 18

What happens when the attractive forces between two atoms are greater than the repulsive forces?

Answer 18

The atoms approach each other and potential energy decreases

This leads to the formation of a stable molecule.

Question 19

What is the bond length of a stable hydrogen molecule (H₂)?

Answer 19

74 pm

The bond length represents the distance at which the forces of attraction and repulsion are balanced.

Question 20

What is bond enthalpy in the context of hydrogen molecules?

Answer 20

The energy released when the bond is formed

The bond enthalpy corresponding to dissociating one mole of H₂ is 435.8 kJ.

Question 21

What is the energy required to dissociate one mole of hydrogen molecules (H₂)?

Answer 21

435.8 kJ mol⁻¹

This energy indicates the stability of the H₂ molecule compared to isolated hydrogen atoms.

Question 22

What concept explains the formation of a covalent bond through the pairing of electrons in atomic orbitals?

Answer 22

Orbital overlap concept

Greater overlap results in stronger covalent bonds.

Question 23

What occurs during the overlapping of atomic orbitals in the formation of a hydrogen molecule?

Answer 23

Partial interpenetration of atomic orbitals

This leads to the pairing of electrons with opposite spins.

Question 24

True or False: The strength of a covalent bond is independent of the extent of orbital overlap.

Answer 24

False

Greater overlap typically leads to a stronger bond.

Question 25

Fill in the blank: The potential energy curve for the formation of H₂ shows a minimum corresponding to the most _____ state.

Answer 25

stable ## Footnote This stable state is where the net attractive and repulsive forces balance.

Question 27

What is formed by the overlapping of atomic orbitals?

Answer 27

Covalent bond ## Footnote The molecule of hydrogen is formed due to the overlap of 1s-orbitals of two H atoms.

Question 28

What is the geometry of the CH4 molecule?

Answer 28

Tetrahedral ## Footnote The HCH bond angles are 109.5°.

Question 29

What shape does the NH3 molecule have?

Answer 29

Pyramidal ## Footnote The valence bond theory helps explain this shape.

Question 30

What theory explains the shape and directional properties of bonds in polyatomic molecules?

Answer 30

Valence bond theory ## Footnote It explains bond formation in terms of overlap and hybridisation of atomic orbitals.

Question 31

What types of overlap can occur when atomic orbitals come close to form a bond?

Answer 31

Positive, negative, or zero overlap ## Footnote The overlap depends on the sign (phase) and orientation of the amplitude of the orbital wave function.

Question 32

What is required for positive overlap to occur?

Answer 32

Same sign (phase) and orientation in space ## Footnote This is essential for the formation of covalent bonds.

Question 33

What are the shapes of CH4, NH3, and H2O molecules?

Answer 33

Tetrahedral, pyramidal, bent ## Footnote These shapes can be explained using valence bond theory.

Question 34

What is the electronic configuration of carbon in its ground state?

Answer 34

(He) 2s² 2p² ## Footnote In the excited state, it becomes (He) 2s' 2p'x' 2p'y' 2p'z.

Question 35

How many C-H bonds are formed in methane (CH4)?

Answer 35

Four C-H bonds ## Footnote Each of the four atomic orbitals of carbon overlaps with the 1s orbitals of four H atoms.

Question 36

What angle do the three p orbitals of carbon form with each other?

Answer 36

90° ## Footnote The HCH angle for these will also be 90°.

Question 37

What is the issue with simple atomic orbital overlap in explaining the geometry of CH4?

Answer 37

It does not account for the tetrahedral HCH angles of 109.5° ## Footnote This indicates that a more complex explanation is needed for the directional characteristics of bonds.

Question 38

Fill in the blank: The criterion of overlap applies uniformly to _______ and heteronuclear diatomic molecules.

Answer 38

homonuclear ## Footnote This principle is essential for the formation of covalent bonds.

Question 40

What are the bond angles in NH3 and H2O molecules?

Answer 40

104.5° in NH3 and 104.5° in H2O ## Footnote The text incorrectly mentions 90°; actual bond angles are approximately 107° for NH3 and 104.5° for H2O.

Question 41

How are covalent bonds classified based on overlapping?

Answer 41

Two types: Sigma (σ) bond and Pi (π) bond ## Footnote This classification is based on the type of overlap between atomic orbitals.

Question 42

What characterizes a Sigma (σ) bond?

Answer 42

Formed by end-to-end (head-on) overlap of bonding orbitals along the internuclear axis ## Footnote Also known as axial overlap.

Question 43

What is s-s overlapping?

Answer 43

Overlap of two half filled s-orbitals along the internuclear axis ## Footnote This type of overlap forms a Sigma bond.

Question 44

Describe s-p overlapping.

Answer 44

Occurs between half filled s-orbitals of one atom and half filled p-orbitals of another atom ## Footnote This is another type of Sigma bond formation.

Question 45

What is p-p overlapping?

Answer 45

Takes place between half filled p-orbitals of two approaching atoms ## Footnote This overlap also contributes to the formation of Sigma bonds.

Question 46

How is a Pi (π) bond formed?

Answer 46

Atomic orbitals overlap such that their axes remain parallel and perpendicular to the internuclear axis ## Footnote This type of bond is formed through sidewise overlapping.

Question 47

What is the strength comparison between Sigma and Pi bonds?

Answer 47

Sigma bonds are stronger than Pi bonds ## Footnote This is due to larger extent of overlapping in Sigma bonds.

Question 48

What is the significance of overlapping in bond strength?

Answer 48

The extent of overlapping determines bond strength ## Footnote Greater overlap leads to stronger bonds.

Question 49

In multiple bonds, what is formed in addition to a Sigma bond?

Answer 49

Pi bond(s) ## Footnote Multiple bonds consist of one Sigma bond and one or more Pi bonds.