Chemistry MCQs for NEET — Practice Questions with Answers

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.'

The NCERT text explicitly mentions, 'In fact the behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.'

The NCERT text explains, 'In addition, the first member of group has only four valence orbitals (2s and 2p) available for bonding, whereas the second member of the groups have nine valence...'. This is due to the absence of d-orbitals in the second period elements (n=2, so only 2s and 2p orbitals are available).

The context states, 'In fact the behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.' Therefore, Li and Mg exhibit a diagonal relationship.

The first element of group 13 (Boron) exhibits anomalous properties due to its small size, large charge/radius ratio, and high electronegativity (II). Like other second-period elements, it only has 2s and 2p orbitals for bonding (III), leading to limited valency and orbital expansion. Unlike its heavier congeners which tend to form more ionic compounds, Boron forms compounds with more covalent character due to its high polarizing power. Thus, statement I is incorrect.