---
title: "Rubidium (Rb) — element 37"
description: "Rubidium is element 37, symbol Rb, atomic weight 85.468. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/rubidium/
site: "At One Place"
---

# Rubidium (Rb)

**Rubidium — element 37: Rb**

85.468 u · Alkali metal · period 5, group 1

Rubidium was the second element found by spectroscopy, a year after Bunsen and Kirchhoff used the same method on caesium: they saw two deep red lines in a mineral water residue that matched nothing known, and named it for the Latin rubidus, deepest red. Rubidium atomic clocks are the workhorses of timekeeping — cheaper and smaller than caesium standards, good to about a second in tens of thousands of years, and sitting in most mobile phone base stations. The first Bose-Einstein condensate, in 1995, was 2,000 rubidium atoms cooled to 170 nanokelvin, and it won the 2001 Nobel Prize in Physics.

## Properties

- **Atomic number:** 37
- **Symbol:** Rb
- **Standard atomic weight:** 85.468 u
- **Category:** Alkali metal
- **Group:** 1
- **Period:** 5
- **Block:** s-block
- **Electron configuration:** [Kr] 5s¹
- **Electrons per shell:** 2, 8, 18, 8, 1
- **Melting point:** 312.46 K — 39.3 °C — 102.8 °F
- **Boiling point:** 961 K — 687.9 °C — 1270.1 °F
- **Density:** 1.53 g/cm³
- **Electronegativity:** 0.82 (Pauling)
- **Oxidation states:** +1
- **Discovered:** 1861 by Robert Bunsen and Gustav Kirchhoff

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹
- **Noble gas shorthand:** [Kr] 5s¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 8 · O: 1

## What it is used for

- Rubidium atomic clocks in phone base stations and GPS receivers
- Laser cooling and Bose-Einstein condensate experiments
- Rubidium-82 for cardiac perfusion PET scans
- Special optical glasses and photocathodes
- Rubidium-strontium dating of very old rock

## Hazards

- Ignites on contact with air and reacts explosively with water

> Melts at 39.3 °C — a hot day, or a warm hand through the glass ampoule.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 35 | Bromine | Br | 79.904 |
| 36 | Krypton | Kr | 83.798 |
| 37 | Rubidium ← | Rb | 85.468 |
| 38 | Strontium | Sr | 87.62 |
| 39 | Yttrium | Y | 88.906 |

## Group 1

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

## Common questions

### What is the atomic number of rubidium?

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

### What is the symbol for rubidium?

Rb.

### What is the electron configuration of rubidium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹, or [Kr] 5s¹ in noble-gas shorthand.

### What is the melting point of rubidium?

312.46 K — 39.3 °C — 102.8 °F

## Related pages

- [Bromine (Br)](https://atoneplace.net/element/bromine/)
- [Krypton (Kr)](https://atoneplace.net/element/krypton/)
- [Strontium (Sr)](https://atoneplace.net/element/strontium/)
- [Yttrium (Y)](https://atoneplace.net/element/yttrium/)
- [Hydrogen (H)](https://atoneplace.net/element/hydrogen/)
- [Lithium (Li)](https://atoneplace.net/element/lithium/)
- [Sodium (Na)](https://atoneplace.net/element/sodium/)
- [Potassium (K)](https://atoneplace.net/element/potassium/)
- [Cesium (Cs)](https://atoneplace.net/element/cesium/)
- [Francium (Fr)](https://atoneplace.net/element/francium/)

## Sources

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