AX is a covalent diatomic molecule where A and X are second row elements of periodic table. Based on Molecular orbital theory, the bond order of AX is 2.5. The total number of electrons in AX is __________. (Round off to the Nearest Integer).

Solution

Chemistry ma’am · 4 steps

A bond order of 2.5 is half-integer, so AX has an odd electron count. Find the familiar molecule.

1. Half-integer clue. Bond order is 1/2(N_b - Nₐ). A value of 2.5 means N_b - Nₐ = 5, an odd number. That only happens when the total electron count is odd.
2. Pick the molecule. NO is the standard heteronuclear molecule of this kind, with both atoms from period 2. N gives 7 electrons and O gives 8, so the total is 7 + 8 = 15.
3. Fill the orbitals. With 15 electrons the order is σ 1s² σ^* 1s² σ 2s² σ^* 2s² σ 2p_z² π 2pₓ² = π 2p_y² π^* 2pₓ¹. That puts 10 electrons in bonding orbitals and 5 in antibonding orbitals.

   Bond order = (N_b - Nₐ)/2 = (10 - 5)/2 = 2.5

4. So. The bond order checks out, so AX is NO and it has 15 electrons in total.

The common slip: 11. Counting only valence electrons (5 + 6) gives 11, but the question asks for the total, which includes the four 1s electrons.

The idea to keep. A half-integer bond order always means an odd number of electrons. Learn the anchors: N₂ (14) has bond order 3, NO (15) has 2.5, and O₂ (16) has 2.

Answer from the NTA final answer key. Solution by PrepWiser.