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Using arrows, write complete orbital diagrams for Scandium, Molybdenum, and Selenium. Box-and-arrow notation represents each orbital as a box and each electron as an arrow (↑ or ↓).
Orbital-diagram rules used in general chemistry
- Aufbau principle: lower-energy orbitals fill before higher-energy orbitals (typical order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, …).
- Pauli exclusion principle: a single orbital holds at most 2 electrons with opposite spins (↑↓).
- Hund’s rule: within a set of degenerate orbitals (p has 3, d has 5), electrons remain unpaired with parallel spins as long as possible.
Color visualization of the valence-region orbital diagrams
Complete orbital diagrams with arrows
Scandium (Sc)
\(Z=21\)
electron configuration: [Ar] 4s² 3d¹
1s
↑↓
2
2s
↑↓
2
2p
↑↓↑↓↑↓
6
3s
↑↓
2
3p
↑↓↑↓↑↓
6
4s
↑↓
2
3d
↑
1
Hund’s rule places the single 3d electron unpaired in one d orbital.
Selenium (Se)
\(Z=34\)
electron configuration: [Ar] 4s² 3d¹⁰ 4p⁴
1s
↑↓
2
2s
↑↓
2
2p
↑↓↑↓↑↓
6
3s
↑↓
2
3p
↑↓↑↓↑↓
6
4s
↑↓
2
3d
↑↓↑↓↑↓↑↓↑↓
10
4p
↑↓↑↑
4
The 4p electrons show two unpaired electrons (a characteristic Hund’s-rule arrangement for p⁴).
Molybdenum (Mo)
\(Z=42\)
electron configuration: [Kr] 5s¹ 4d⁵
1s
↑↓
2
2s
↑↓
2
2p
↑↓↑↓↑↓
6
3s
↑↓
2
3p
↑↓↑↓↑↓
6
4s
↑↓
2
3d
↑↓↑↓↑↓↑↓↑↓
10
4p
↑↓↑↓↑↓
6
5s
↑
1
4d
↑↑↑↑↑
5
Mo is a standard electron-configuration exception: the arrangement [Kr] 5s¹ 4d⁵ is favored over [Kr] 5s² 4d⁴ because a half-filled d subshell (d⁵) is especially stable.
Compact summary
| Element | \(Z\) | Electron configuration (shorthand) | Key orbital-diagram feature |
|---|---|---|---|
| Sc | 21 | [Ar] 4s² 3d¹ | One unpaired electron in 3d; 4s is paired. |
| Se | 34 | [Ar] 4s² 3d¹⁰ 4p⁴ | Two unpaired electrons in 4p (Hund’s-rule pattern for p⁴). |
| Mo | 42 | [Kr] 5s¹ 4d⁵ | Exception with a half-filled 4d subshell; 5s has one electron. |
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