Electron configuration quiz | Learning Lab - Free Printable

To determine which orbital diagram is correct, we need to apply three main rules for filling electron orbitals:

1. Aufbau Principle: Electrons fill lower energy orbitals first (1s → 2s → 2p → 3s → 3p).
2. Pauli Exclusion Principle: An orbital can hold a maximum of 2 electrons, and they must have opposite spins (one up, one down). You cannot have two arrows pointing in the same direction in the same box.
3. Hund’s Rule: When filling orbitals of the same energy (like the three boxes in 2p or 3p), electrons occupy empty orbitals singly with parallel spins (all up) before pairing up.

Let's analyze each option step by step:

Option a:
- Look at the 3p subshell. It has 4 electrons.
- According to Hund's Rule, the first three electrons should go into separate boxes with upward spins. The fourth electron should then pair up with the first one (downward spin).
- In diagram a, the electrons are arranged as: [↑↓] [↑] [↑]. This follows Hund's Rule correctly.
- Let's check the rest:
- 1s, 2s, 2p, 3s are all fully filled with paired electrons (↑↓). This is correct.
- Total electrons: $2 + 2 + 6 + 2 + 4 = 16$. This corresponds to Sulfur (S).
- The arrangement looks valid so far.

Option b:
- Look at the 2s orbital. It has only one electron (↑).
- However, the 2p orbitals are already being filled.
- According to the Aufbau Principle, the lower energy 2s orbital must be completely full (↑↓) before any electrons enter the higher energy 2p orbitals.
- Therefore, diagram b is incorrect.

Option c:
- Look at the 3p subshell. It has 4 electrons.
- The arrangement shown is: [↑] [↑↓] [ ].
- This violates Hund's Rule. Electrons should not pair up in the second box while the third box is still empty. They must spread out first: [↑↓] [↑] [↑] or [↑] [↑] [↑] then pair. Specifically, you fill all singly first.
- Also, look closely at the 2p subshell. It is full (6 electrons). That's fine.
- But wait, look at the 3p again. The middle box has paired electrons (↑↓), but the last box is empty. This is a violation of Hund's Rule. Electrons repel each other, so they prefer to be in separate orbitals if possible.
- Therefore, diagram c is incorrect.

Option d:
- Look at the 3p subshell. It has 4 electrons.
- The arrangement shown is: [↑] [↑] [↓].
- This violates Hund's Rule regarding spin. The first three electrons should all have the *same* spin (usually drawn as up). The fourth electron pairs up with an opposite spin.
- Here, the third electron is pointing down while the first two are up, and no pairing has occurred yet. This is not the lowest energy state. The correct ground state would be [↑↓] [↑] [↑].
- Therefore, diagram d is incorrect.

Re-evaluating Option a:
- 1s: ↑↓ (Full)
- 2s: ↑↓ (Full)
- 2p: ↑↓ ↑↓ ↑↓ (Full)
- 3s: ↑↓ (Full)
- 3p: ↑↓ ↑ ↑ (4 electrons)
- First 3 electrons go in separate boxes: ↑ ↑ ↑
- 4th electron pairs in the first box: ↑↓ ↑ ↑
- This matches diagram a perfectly.

Therefore, a is the only correct orbital diagram representing the ground state of an atom (specifically Sulfur).

Final Answer: a