The electron configuration 1S$^2$ 2S$^2$ 2P$_x$$^2$ contravenes the

Analyze the question

The configuration 1s² 2s² 2pₓ² has 6 electrons (carbon). The two 2p electrons are both placed in the same orbital (2pₓ), even though 2p_y and 2p_z are empty. We need to find which rule this breaks.

Concept: the three rules for filling orbitals

• Aufbau principle: electrons fill the lowest-energy orbitals first (1s → 2s → 2p → 3s...).
• Pauli's exclusion principle: no two electrons in an atom can have the same four quantum numbers, so an orbital holds a maximum of 2 electrons with opposite spins.
• Hund's rule: electrons occupy degenerate orbitals (such as the three 2p orbitals) singly first, with parallel spins, before any pairing begins.

Solution

1. 1s² and 2s² are filled in the correct order, so Aufbau is obeyed.
2. 2pₓ² puts two electrons in one orbital. This is allowed by Pauli, as long as their spins are opposite.
3. The octet rule concerns bonding and valence shells, not ground-state orbital filling, so it is not relevant here.
4. The problem is that 2p_y and 2p_z are empty while 2pₓ is doubly occupied. The correct configuration is 1s² 2s² 2pₓ¹ 2p_y¹, with the two electrons unpaired in separate p orbitals. Pairing before every degenerate orbital has one electron breaks Hund's rule.

Answer: D. Hund's rule

Tip: Think of Hund's rule as "bus seat" behavior. People take an empty seat of their own first and only sit together when there are no empty seats left. If you see pairing in an orbital set while other orbitals in the same subshell are empty, the answer is Hund's rule.