The number of unpaired electrons in an atom of an element 8Q is _______?

Analyze the question: Element ₈Q has atomic number 8, so it is oxygen with 8 electrons. We need the number of unpaired electrons in its ground state.

Concept: Electrons fill orbitals in order of increasing energy (Aufbau principle). Within a subshell, electrons occupy separate orbitals singly first, with parallel spins, before pairing up (Hund's rule).

Solution:

1. Electron configuration of oxygen: 1s² 2s² 2p⁴

2. 1s² is a full orbital, so both electrons are paired.
3. 2s² is a full orbital, so both electrons are paired.
4. 2p⁴: the 2p subshell has three orbitals (2pₓ, 2p_y, 2p_z). By Hund's rule, the first three electrons go in singly, and the fourth pairs up with one of them:

• 2pₓ: ↑↓ (paired)
• 2p_y: ↑ (unpaired)
• 2p_z: ↑ (unpaired)

5. Number of unpaired electrons = 2

This also matches oxygen's valency of 2. It forms two bonds, as in H₂O.

Answer: The correct option is A. 2.

Tip: For p-block elements, count the p electrons. For p¹ to p³ the unpaired electrons equal the p electron count. For p⁴ to p⁶ the unpaired electrons are 6 minus the p count. Here p⁴ gives 6 − 4 = 2.