Question

Explain the bond formation between $N{H_3}$ and $C{H_4}$$(VBT)$

Answer 1

Hint: Electrons in a molecule occupy atomic orbitals rather than molecular orbitals. The atomic orbitals overlap on the bond formation and the larger the overlap the stronger the bond. Valence bond theory is typically easier to employ in ground state molecules.

Complete answer:
According to the valence bond theory a covalent bond is formed between two atoms by the overlap of half-filled valence atomic orbitals of each atom containing one unpaired electron. Each of these valence bond structures represents a specific Lewis structure.
The bonds in a methane $(C{H_4})$ molecule are formed by four separate but equivalent orbitals, a single $2s$ and three $2p$ orbitals of the carbon hybridize into four $s{p^3}$ orbitals. . Carbon’s four valence electrons occupy the orbitals singly with parallel spins as dictated by Hund’s rule. With this electron configuration, carbon has four half-filled orbitals and can form four bonds with four hydrogen atoms. The geometry of the overlapping orbitals is tetrahedral, with angles of ${109.5^ \circ }$ between the orbitals, so the resulting geometry of the molecule is tetrahedral, with ${109.5^ \circ }$ bond angles.
In the ammonia molecule$(N{H_3})$, $2s$ and $2p$ orbitals create four $s{p^3}$ hybrid orbitals, one of which is occupied by a lone pair of electrons. The presence of the lone pair lowers the tendency of nitrogen’s orbitals to hybridize. Therefore the bond angle in $N{H_3}$is ${107^ \circ }$, a bit closer to the unhybridized $p$ orbital bond angle of ${90^ \circ }$

Note:
Valence bond theory is one of the two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. In contrast, molecular orbital theory has orbitals that cover the whole molecule.