---
title: "Potassium (K) — element 19"
description: "Potassium is element 19, symbol K, atomic weight 39.098. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/potassium/
site: "At One Place"
---

# Potassium (K)

**Potassium — element 19: K**

39.098 u · Alkali metal · period 4, group 1

Every person is measurably radioactive because of potassium-40, which makes up about 0.012% of natural potassium and has a half-life of 1.25 billion years — an adult body emits roughly 4,000 to 5,000 decays a second from it. That same decay to argon-40 is the basis of potassium-argon dating, which is how the age of most volcanic rock is established. A banana contains enough potassium to be detectable by a sensitive radiation monitor, which is where the informal "banana equivalent dose" comes from.

## Properties

- **Atomic number:** 19
- **Symbol:** K
- **Standard atomic weight:** 39.098 u
- **Category:** Alkali metal
- **Group:** 1
- **Period:** 4
- **Block:** s-block
- **Electron configuration:** [Ar] 4s¹
- **Electrons per shell:** 2, 8, 8, 1
- **Melting point:** 336.53 K — 63.4 °C — 146.1 °F
- **Boiling point:** 1,032 K — 758.9 °C — 1397.9 °F
- **Density:** 0.89 g/cm³
- **Electronegativity:** 0.82 (Pauling)
- **Oxidation states:** +1
- **Discovered:** 1807 by Humphry Davy

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
- **Noble gas shorthand:** [Ar] 4s¹
- **Shells:** K: 2 · L: 8 · M: 8 · N: 1

## What it is used for

- Potash fertiliser — the K in N-P-K
- Potassium hydroxide in soft soaps and alkaline batteries
- Potassium nitrate in gunpowder and in curing salts
- Salt substitutes for people on low-sodium diets
- Potassium chloride solutions in medicine, to correct low blood potassium

## Hazards

- Reacts violently with water, usually igniting the hydrogen it releases
- Old potassium metal forms shock-sensitive peroxides on its surface

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 17 | Chlorine | Cl | 35.45 |
| 18 | Argon | Ar | 39.95 |
| 19 | Potassium ← | K | 39.098 |
| 20 | Calcium | Ca | 40.078 |
| 21 | Scandium | Sc | 44.956 |

## Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Potassium sits in group 1 with hydrogen, lithium, sodium, rubidium, cesium, francium.

## Common questions

### What is the atomic number of potassium?

19. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for potassium?

K.

### What is the electron configuration of potassium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹, or [Ar] 4s¹ in noble-gas shorthand.

### What is the melting point of potassium?

336.53 K — 63.4 °C — 146.1 °F

## Related pages

- [Chlorine (Cl)](https://atoneplace.net/element/chlorine/)
- [Argon (Ar)](https://atoneplace.net/element/argon/)
- [Calcium (Ca)](https://atoneplace.net/element/calcium/)
- [Scandium (Sc)](https://atoneplace.net/element/scandium/)
- [Hydrogen (H)](https://atoneplace.net/element/hydrogen/)
- [Lithium (Li)](https://atoneplace.net/element/lithium/)
- [Sodium (Na)](https://atoneplace.net/element/sodium/)
- [Rubidium (Rb)](https://atoneplace.net/element/rubidium/)
- [Cesium (Cs)](https://atoneplace.net/element/cesium/)
- [Francium (Fr)](https://atoneplace.net/element/francium/)

## Sources

- Official vendor product documentation — At One Place (/about/)