Explain: He2 is not formed but He2+ is. Explain using Molecular Orbital Theory.

Q.1. He2 is not formed but He2+ is. Explain using Molecular Orbital Theory.

In molecular orbital theory, the formation of molecules is explained by the overlap of atomic orbitals to create bonding and antibonding molecular orbitals. Helium, being an inert gas, typically does not form stable molecules under normal conditions because its electrons fill the 1s orbital completely, making it energetically unfavorable to form bonds.

In the case of helium, which has two electrons, the molecular orbital theory predicts that when two helium atoms approach each other, the two 1s orbitals combine to form a bonding molecular orbital (σ1s) and an antibonding molecular orbital (σ*1s). However, both electrons in the helium atoms occupy the 1s orbital, so when the orbitals combine, the resulting energy levels of the bonding and antibonding orbitals are both fully occupied. This leads to no net stabilization or destabilization, resulting in the lack of stable He2 molecule formation.

MOT: Helium molecules

However, helium can form a stable ion, He2+, under certain conditions. When one electron is removed from the He2 molecule, the resulting He2+ ion has a missing electron in the antibonding molecular orbital. This makes the remaining electron more stable than it was in the neutral He2 molecule. Therefore, He2+ can be formed because removing an electron from the antibonding orbital stabilizes the remaining electron, leading to a stable ion.

Q.2 . Discuss the molecular geometries of the following :

i) BCl3

ii) PCl5

    (Atomic number: B = 5, P = 15)

Ans: To determine the molecular geometries of BCl₃ and PCl₅, we can use the VSEPR (Valence Shell Electron Pair Repulsion) theory. This theory predicts the shapes of molecules based on the repulsion between electron pairs around the central atom. The shapes are determined by the number of bonding pairs and lone pairs around the central atom.

i) BCl₃ (Boron Trichloride):

Boron (B) has three valence electrons, and chlorine (Cl) has seven. BCl₃ has three chlorine atoms bonded to the central boron atom, resulting in three bonding pairs.

According to VSEPR theory, the molecule adopts a trigonal planar geometry. The bond angles between the chlorine atoms are approximately 120 degrees.

ii) PCl₅ (Phosphorus Pentachloride):

Phosphorus (P) has five valence electrons, and each chlorine (Cl) has seven. PCl₅ has five chlorine atoms bonded to the central phosphorus atom, resulting in five bonding pairs.

According to VSEPR theory, the molecule adopts a trigonal bipyramidal geometry. The arrangement includes three equatorial chlorine atoms in a trigonal plane and two axial chlorine atoms above and below this plane. The bond angles between the equatorial chlorine atoms are approximately 120 degrees, and the bond angles between the axial and equatorial chlorine atoms are around 90 degrees.

Geometry and structure of BCl3 and PCl5

In summary:

i) BCl₃ has a trigonal planar molecular geometry with bond angles of approximately 120 degrees.

ii) PCl₅ has a trigonal bipyramidal molecular geometry with bond angles of approximately 120 degrees (equatorial) and 90 degrees (axial).