Molybdenum (Mo)
[Kr] 4d⁵ 5s¹, an exception. 6 unpaired electrons, period 5, group 6, d block.
Element or ion
Type a symbol, a name or an atomic number, add a charge such as Fe3+ or Cl-, or tap an element in the table. Orbitals fill from m = −l up, spin +½ first.
MoMolybdenum, Z = 42, 95.95 u
[Kr] 4d5 5s1
In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹
- Blockd block
- Period5
- Group6
- Valence6
- Unpaired6
- MagneticPara
Last electron 4d
- n4shell
- l2d subshell
- m+2orbital
- s+½spin up
Why Mo is an exception
Plain filling would give [Kr] 4d⁴ 5s². A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.
Step by step
- Molybdenum has 42 electrons.
- Fill in (n + l) order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d⁴.
- That gives [Kr] 4d⁴ 5s², but the accepted configuration is [Kr] 4d⁵ 5s¹. A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.
- Write the subshells in order of n: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹.
- Replace the krypton core with [Kr]: [Kr] 4d⁵ 5s¹.
Molybdenum (Mo): the numbers
| Atomic number | 42 |
|---|---|
| Atomic mass | 95.95 u |
| Period | 5 |
| Group | 6 |
| Block | d block |
| Valence electrons | 6 |
| Unpaired electrons | 6 |
| Magnetic | paramagnetic |
| Last electron | 4d: n = 4, l = 2, m = +2, s = +1/2 |
Molybdenum (Mo): quick answers
What is the electronic configuration of molybdenum?
Molybdenum has 42 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹, or [Kr] 4d⁵ 5s¹ in short.
Why is molybdenum [Kr] 4d⁵ 5s¹ and not [Kr] 4d⁴ 5s²?
A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.
What are the quantum numbers of the last electron of molybdenum?
The last electron goes into 4d: n = 4, l = 2, m = +2, s = +1/2. Orbitals fill from m = −l up, spin +½ first.