The Electron Configuration Practice Problems | Channels for Pearson+ - Free Printable

The image shows four electron configuration diagrams (A, B, C, and D) for an atom, using arrow notation to represent electrons in orbitals. Each box represents an orbital, and the arrows (↑↓) indicate electrons with opposite spins.

We are likely being asked to identify which configuration follows the rules of electron filling, specifically:

- Aufbau Principle: Electrons fill orbitals starting from the lowest energy level.
- Pauli Exclusion Principle: No orbital can have more than two electrons, and they must have opposite spins (↑↓).
- Hund’s Rule: When filling degenerate orbitals (like 2p), electrons will occupy each orbital singly before pairing up, and all unpaired electrons will have the same spin.

Let’s analyze each option:

---

Option A

```
1s: ↑↓
2s: ↑↓
2p: ↑↓ ↑↓ ↑
```

- 1s: 2 electrons → OK (full)
- 2s: 2 electrons → OK (full)
- 2p: Three orbitals:
- First: ↑↓ (2 electrons)
- Second: ↑↓ (2 electrons)
- Third: ↑ (1 electron)

But wait — this has 5 electrons in the 2p subshell? Let's count:
- 1s² → 2
- 2s² → 2
- 2p⁵ → 5
Total = 9 electrons

But the 2p subshell has three orbitals, and here we have:
- Two orbitals filled with paired electrons (4 electrons),
- One orbital with one electron.

This is not following Hund’s rule because electrons should occupy each orbital singly before pairing.

In a correct 2p⁵ configuration, you'd have:
- All three orbitals have at least one electron,
- Then two of them are paired.

So: ↑↓, ↑↓, ↑ → but that’s valid only if it's p⁵.

Wait — actually, p⁵ is allowed, and it does follow Hund’s rule: three orbitals, five electrons → two orbitals are paired, one has a single electron.

But in this diagram:
- The first two 2p orbitals are both paired (each has ↑↓), and the third has ↑.
- That means two orbitals are filled, one has one electron.

But this is acceptable for p⁵ — total 5 electrons.

Wait — is it?

Yes! For p⁵, the configuration is:
- Orbital 1: ↑↓
- Orbital 2: ↑↓
- Orbital 3: ↑

That's fine — two pairs and one unpaired.

But is Hund’s rule violated?

No — Hund’s rule says: maximize unpaired electrons with parallel spins when filling degenerate orbitals.

But once pairing begins, it's okay.

For p⁵: You have to pair up two orbitals, leave one unpaired.

So yes, this is valid.

But let's check the order of filling.

All orbitals are filled in order: 1s → 2s → 2p → correct.

So A looks okay.

But let's compare to others.

---

Option B

```
1s: ↑↓
2s: ↑
2p: ↑↓ ↑↓ ↑↓
```

- 1s: 2 electrons → OK
- 2s: 1 electron → not full
- 2p: all three orbitals have ↑↓ → 6 electrons

Total electrons: 2 + 1 + 6 = 9

But the 2s orbital is not filled while 2p is completely filled? That violates the Aufbau principle.

Electrons must fill lower energy levels first.

2s is lower energy than 2p.

So you cannot have 2s half-filled while 2p is full.

This is invalid.

So B is incorrect.

---

Option C

```
1s: ↑↓
2s: ↑↓
2p: ↑↑ ↑↓ ↑
```

- 1s: 2 → OK
- 2s: 2 → OK
- 2p:
- First orbital: ↑↑ → two electrons with same spin → violates Pauli Exclusion Principle

Pauli says: no two electrons in the same orbital can have the same spin.

Here, both arrows are ↑ → not allowed.

So C is invalid.

---

Option D

```
1s: ↑
2s: ↑↓
2p: ↑↓ ↑↓ ↑↓
```

- 1s: only one electron → not full
- 2s: full → 2 electrons
- 2p: full → 6 electrons

Total: 1 + 2 + 6 = 9 electrons

But 1s is not filled, yet higher orbitals (2s, 2p) are filled.

This violates Aufbau principle — you must fill 1s completely before moving to 2s.

So D is invalid.

---

Final Analysis:

Only Option A satisfies:

- Aufbau principle: 1s → 2s → 2p
- Pauli exclusion: no orbital has >2 electrons or same-spin pairs
- Hund’s rule: for 2p⁵, two orbitals paired, one unpaired — acceptable

Even though 2p has two paired orbitals and one unpaired, that’s correct for p⁵.

But wait — is there any issue with the spin alignment?

In Option A, the 2p orbitals show:

- First: ↑↓
- Second: ↑↓
- Third: ↑

All unpaired electrons are up — that’s fine, as long as the pairing is correct.

And since the electrons are in different orbitals, their spins don’t need to be aligned unless they're unpaired.

But for Hund’s rule, when filling degenerate orbitals, unpaired electrons should have parallel spins.

But here, we have only one unpaired electron in 2p, so no issue.

So yes, A is correct.

---

✔ Correct Answer: A

It correctly represents the electron configuration for an atom with 9 electrons (e.g., fluorine: 1s² 2s² 2p⁵), following all quantum rules.

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Summary:

| Option | Valid? | Reason |
|-------|--------|--------|
| A | ✔ Yes | Follows Aufbau, Pauli, Hund |
| B | ✘ No | 2s not filled before 2p |
| C | ✘ No | Violates Pauli (same spin in orbital) |
| D | ✘ No | 1s not filled before higher orbitals |

Answer: A ✔