What is the electron configuration of potassium?

Potassium (K, Z=19) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Its 19 electrons fill six subshells across four principal shells.

What is the noble-gas shorthand for potassium?

The condensed notation is [Ar] 4s¹, where [Ar] represents the filled argon core (1s² 2s² 2p⁶ 3s² 3p⁶) shared with argon (Z=18).

Why does 4s fill before 3d in potassium?

In the neutral atom, the 4s orbital lies slightly lower in energy than the 3d orbitals. The Aufbau principle places electrons in the lowest available energy level first, so 4s receives the 19th electron rather than 3d.

Why does potassium form a +1 ion?

Potassium loses its single 4s¹ valence electron to reach the stable [Ar] noble-gas configuration, forming K⁺. The energy cost of removing one electron is far less than removing a second from the filled argon core.

How many valence electrons does potassium have?

Potassium has 1 valence electron — the single electron in the 4s subshell. This is the defining feature of all Group 1 alkali metals.

What block and period is potassium in?

Potassium is in Period 4, Group 1 (IA), and belongs to the s-block of the periodic table. It is the first element of Period 4.

What is the full quantum number set for the 4s electron?

For the lone 4s¹ electron: principal quantum number n=4, azimuthal l=0, magnetic mₗ=0, spin mₛ=+½ (spin up by convention).

Potassium Electron Configuration

Name: Potassium Electron Configuration
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Author: Nham Vu

Interactive reference for potassium's electron configuration (1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹), orbital diagram, quantum numbers, and key atomic properties.

Z = 19 K Potassium

Potassium — Electron Configuration

Atomic number 19 · Alkali metal · Period 4, Group 1 · s-block

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ [Ar] 4s¹ 19 electrons 1 valence e⁻

Subshell Breakdown

Subshell	Type	Electrons	Max Capacity	Notation
1s	s orbital, shell n=1	2	2	1s²
2s	s orbital, shell n=2	2	2	2s²
2p	p orbitals, shell n=2	6	6	2p⁶
3s	s orbital, shell n=3	2	2	3s²
3p	p orbitals, shell n=3	6	6	3p⁶
4s	s orbital, shell n=4 (valence)	1	2	4s¹
Total		19

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

All subshells written explicitly. Total: 19 electrons.

Noble-Gas Shorthand

[Ar] 4s¹

[Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶ (the filled argon core, Z=18).

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 8 / 18 electrons (44%)

Shell 4 (n=4) — 4s¹ 1 / 32 electrons (3%)

Shell 3 can hold up to 18 electrons (3s + 3p + 3d); potassium uses only 8. The 4s fills before 3d because 4s is lower in energy for Z=19.

Why 4s Fills Before 3d

For elements from potassium (Z=19) through calcium (Z=20), the 4s orbital is slightly lower in energy than the 3d orbitals. The 19th electron of potassium therefore occupies 4s rather than 3d. Once the 3d subshell begins filling in the transition metals (Z=21 onward), orbital energies shift — but for potassium specifically, the configuration is unambiguously [Ar] 4s¹.

Orbital Box Diagram

2 electrons (paired) — filled

6 electrons across 3 orbitals — filled

2 electrons (paired) — filled

6 electrons across 3 orbitals — filled (= argon core ends here)

1 electron (unpaired, spin up) — lone valence electron

Each box represents one orbital. Opposing arrows represent paired electrons (spin +½ and −½). The single 4s¹ electron is unpaired — it is the first electron to be lost when potassium reacts, forming K⁺.

Quantum Numbers for All 19 Electrons

Electron #	n	l	mₗ	mₛ	Subshell
1	1	0	0	+½	1s
2	1	0	0	−½	1s
3	2	0	0	+½	2s
4	2	0	0	−½	2s
5	2	1	-1	+½	2p
6	2	1	-1	−½	2p
7	2	1	0	+½	2p
8	2	1	0	−½	2p
9	2	1	1	+½	2p
10	2	1	1	−½	2p
11	3	0	0	+½	3s
12	3	0	0	−½	3s
13	3	1	-1	+½	3p
14	3	1	-1	−½	3p
15	3	1	0	+½	3p
16	3	1	0	−½	3p
17	3	1	1	+½	3p
18	3	1	1	−½	3p
19	4	0	0	+½	4s

n = principal quantum number; l = azimuthal (l=0 → s, l=1 → p); mₗ = magnetic; mₛ = spin. Electron 19 (4s, highlighted) is the lone valence electron.

Summary