NEET Practice Questions (MCQs) with Answers & Solutions

The text states: 'Because of orbital overlap the electron density between the nuclei increases which helps in bringing them closer.'

The context states: 'For explaining the characteristic shapes of polyatomic molecules Pauling introduced the concept of hybridisation of atomic orbitals.'

The text explicitly mentions: 'Valence bond theory was introduced by Heitler and London (1927) and developed further by Pauling and others.'

The passage lists: 'A discussion of the valence bond theory is based on the knowledge of atomic orbitals, electronic configurations of elements (Units 2), the overlap criteria of atomic orbitals, the hybridization of atomic orbitals and the principles of variation and superposition.' Lattice energy is related to ionic compounds, not explicitly the basis for VB theory presented here.

The text details the formation of $H_2$: 'At the equilibrium inter-nuclear distance (bond distance) the energy touches a minimum. Any attempt to bring the nuclei still closer results in a sudden increase in energy and consequent destabilization of the molecule.'

The text states: 'sp, $sp^2$, $sp^3$ hybridizations of atomic orbitals of Be, B, C, N and O are used to explain the formation and geometrical shapes of molecules like $BeCl_2$, $BCl_3$, $CH_4$, $NH_3$ and $H_2O$.'

The text mentions: 'As we know that Lewis approach helps in writing the structure of molecules but it fails to explain the formation of chemical bond. It also does not give any reason for the difference in bond dissociation enthalpies and bond lengths...'

The text states: 'Similarly the VSEPR theory gives the geometry of simple molecules but theoretically, it does not explain them and also it has limited applications. To overcome these limitations the two important theories based on quantum mechanical principles are introduced. These are valence bond (VB) theory and molecular orbital (MO) theory.' VB theory discusses bond formation 'in terms of overlap of orbitals.'

According to the NCERT text, 'The anomalous behaviour is attributed to their small size, large charge/radius ratio and high electronegativity of the elements. In addition, the first member of group has only four valence orbitals (2s and 2p) available for bonding'. Second-period elements do not possess d-orbitals in their valence shell, which is a characteristic of later period elements. The absence of d-orbitals contributes to their anomalous behavior, not their presence.

The NCERT text states, 'For example, lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds.'