Question

The number of non-bonding electrons present in the frontier molecular orbitals of HF is

• A. 10
• B. 4
• C. 6
• D. 8

Hint:

Lone pairs on atoms in a molecule contribute to non-bonding electrons in the frontier molecular orbitals.

Answer 1

The Correct Option is B

To determine the number of non-bonding electrons present in the frontier molecular orbitals of HF (hydrogen fluoride), we need to understand the molecular orbital theory as applied to diatomic molecules like HF.

In HF, the bond is formed between hydrogen and fluorine. The fluorine atom is more electronegative and therefore attracts the bonding electrons more strongly than hydrogen. Let's consider the electron configuration for each atom:

• Hydrogen (H): 1s¹
• Fluorine (F): 1s² 2s² 2p⁵

In the molecular orbital (MO) theory, when HF is formed, the 1s orbital of hydrogen and the 2p orbitals of fluorine overlap to form a bonding molecular orbital and a non-bonding molecular orbital. The 1s² of fluorine and the 2s² of fluorine do not participate in bonding and are considered non-bonding electron pairs.

Reviewing the MOs:

• The bonding orbital involves the overlap of the hydrogen 1s and one of the fluorine 2p orbitals.
• The remaining 2p orbitals on fluorine, not involved in bonding, remain as non-bonding orbitals.

Therefore, the non-bonding electrons present in the frontier molecular orbitals of HF are those remaining in the fluorine 2p orbitals that do not participate in bonding. Fluorine's 2p orbitals initially have 5 electrons, 2 of which are used in bonding. Thus, 3 electrons are left as non-bonding. However, considering common practice for molecular frontier orbitals interaction:

• One pair (2 electrons) from the fluorine 2p remains non-bonding after forming sigma bonds.

Thus, the number of non-bonding electrons present in the frontier molecular orbitals is 4 (as there are two non-bonding pairs on fluorine).

Hence, the correct answer is 4.