---
title: "Beryllium (Be) — element 4"
description: "Beryllium is element 4, symbol Be, atomic weight 9.0122. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/beryllium/
site: "At One Place"
---

# Beryllium (Be)

**Beryllium — element 4: Be**

9.0122 u · Alkaline earth metal · period 2, group 2

Beryllium is stiffer than steel at a quarter of the weight, which is why the James Webb Space Telescope's 18 mirror segments are made of it — it barely changes shape as it cools to 40 K. It is also one of the more insidious industrial poisons: inhaled dust causes chronic beryllium disease in a minority of exposed people, a permanent lung condition driven by an immune response, and the susceptible fraction is largely genetic. Beryllium-8, the isotope two helium nuclei would form, falls apart in about 10⁻¹⁶ seconds, and the fact that stars have to build carbon through a fleeting three-body collision instead is one of the sharper coincidences in physics.

## Properties

- **Atomic number:** 4
- **Symbol:** Be
- **Standard atomic weight:** 9.0122 u
- **Category:** Alkaline earth metal
- **Group:** 2
- **Period:** 2
- **Block:** s-block
- **Electron configuration:** [He] 2s²
- **Electrons per shell:** 2, 2
- **Melting point:** 1,560 K — 1286.8 °C — 2348.3 °F
- **Boiling point:** 2,744 K — 2470.8 °C — 4479.5 °F
- **Density:** 1.85 g/cm³
- **Electronegativity:** 1.57 (Pauling)
- **Oxidation states:** +2
- **Discovered:** 1798 by Louis-Nicolas Vauquelin

## Electron configuration

- **Full:** 1s² 2s²
- **Noble gas shorthand:** [He] 2s²
- **Shells:** K: 2 · L: 2

## What it is used for

- X-ray tube windows, because it is nearly transparent to X-rays
- Beryllium-copper tools that will not spark in a gas atmosphere
- Mirrors for the James Webb Space Telescope
- Neutron reflectors and moderators in research reactors
- Stiff, light structures in satellites and inertial guidance systems

## Hazards

- Inhaled dust or fume causes chronic beryllium disease, which is incurable
- Classified as a human carcinogen
- Machining, grinding and welding are the dangerous operations, not handling solid metal

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 2 | Helium | He | 4.0026 |
| 3 | Lithium | Li | 6.94 |
| 4 | Beryllium ← | Be | 9.0122 |
| 5 | Boron | B | 10.81 |
| 6 | Carbon | C | 12.011 |

## Group 2

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Beryllium sits in group 2 with magnesium, calcium, strontium, barium, radium.

## Common questions

### What is the atomic number of beryllium?

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

### What is the symbol for beryllium?

Be.

### What is the electron configuration of beryllium?

1s² 2s², or [He] 2s² in noble-gas shorthand.

### What is the melting point of beryllium?

1,560 K — 1286.8 °C — 2348.3 °F

## Related pages

- [Helium (He)](https://atoneplace.net/element/helium/)
- [Lithium (Li)](https://atoneplace.net/element/lithium/)
- [Boron (B)](https://atoneplace.net/element/boron/)
- [Carbon (C)](https://atoneplace.net/element/carbon/)
- [Magnesium (Mg)](https://atoneplace.net/element/magnesium/)
- [Calcium (Ca)](https://atoneplace.net/element/calcium/)
- [Strontium (Sr)](https://atoneplace.net/element/strontium/)
- [Barium (Ba)](https://atoneplace.net/element/barium/)
- [Radium (Ra)](https://atoneplace.net/element/radium/)

## Sources

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