Question

Main axis of diatomic molecule is Z. the orbitals ${p_x}$ and ${p_y}$ overlap to form:
A.$\pi$ -molecular orbital
B.$\sigma$ -molecular orbital
C.$\delta$ -molecular orbital
D.no bond is formed.

Answer 1

For almost every covalent bond we can draw the Lewis structure, predict the electron-pair geometry, predict the molecular geometry and come close to predict bond angles. Molecular orbital theory describes the distribution of electrons in molecules.

Complete answer:
There are two types of molecular orbitals that can form from the overlap of two atomic s orbitals on adjacent atoms. The in-phase combination produces a lower energy ${\sigma _s}$ molecular orbital (read as sigma-s) and the out-of-phase addition produces a higher energy ${\sigma ^*}_s$ molecular orbital (read as sigma-s-star). Addition of electrons to the ${\sigma _s}$ orbitals creates a force that holds the two nuclei together, so we call these orbitals as bonding orbitals. In ${\sigma ^*}_s$ the attractive force between the nuclei and these electrons pulls the two nuclei apart. Hence, these orbitals are called antibonding orbitals.
When p orbitals overlap end to end, they create $\sigma$and ${\sigma ^*}$ orbitals. If two atoms are located along x-axis, the two ${p_x}$ orbitals overlap end to end and forms ${\sigma _{px}}$ (bonding) and ${\sigma ^*}_{px}$ (antibonding). The side by side overlap of two p orbitals give rise to a pi $(\pi )$bonding molecular orbital and a ${\pi ^*}$ antibonding molecular orbital.
In molecular orbitals of diatomic molecules, each atom also have two sets of p orbitals oriented side by side $({p_y}_{}an{d_{}}{p_z})$, so there are four atomic orbitals combine pairwise to create two $\pi$ orbitals and two ${\pi ^*}$ orbitals. The ${\pi _{py}}$ and ${\pi _{pz}}$ orbitals are identical and have the same energy, they are degenerate orbitals.
Hence the correct answer is option A.

Note:
The ${\pi _{py}}$ and ${\pi _{py}}^*$ orbitals are right angles to the ${\pi _{pz}}$ and ${\pi _{pz}}^*$. The ${\pi _{py}}^*$ and ${\pi ^*}_{px}$ are also degenerated and identical except for their orientation. A total of six molecular orbitals result from the combination of the six atomic p orbitals in two atoms: ${\sigma _{px}}$ and ${\sigma ^*}_{px}$, ${\pi _{py}}$ and ${\pi _{py}}^*$ , ${\pi _{pz}}$ and ${\pi _{pz}}^*$ .