---
title: "Selenium (Se) — element 34"
description: "Selenium is element 34, symbol Se, atomic weight 78.971. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/selenium/
site: "At One Place"
---

# Selenium (Se)

**Selenium — element 34: Se**

78.971 u · Nonmetal · period 4, group 16

Selenium's electrical resistance drops when light falls on it, a property discovered in 1873 that led directly to the first light meters, the first photocells and eventually to xerography — every photocopier before the 1980s worked on a selenium drum. The gap between a necessary dose and a toxic one is unusually narrow: about 55 micrograms a day is required and roughly 400 is the safe upper limit. In parts of China with selenium-poor soil, deficiency causes a heart disease called Keshan disease, while a few selenium-rich areas produce hair loss and brittle nails from excess.

## Properties

- **Atomic number:** 34
- **Symbol:** Se
- **Standard atomic weight:** 78.971 u
- **Category:** Nonmetal
- **Group:** 16
- **Period:** 4
- **Block:** p-block
- **Electron configuration:** [Ar] 4s² 3d¹⁰ 4p⁴
- **Electrons per shell:** 2, 8, 18, 6
- **Melting point:** 493.65 K — 220.5 °C — 428.9 °F
- **Boiling point:** 958 K — 684.9 °C — 1264.7 °F
- **Density:** 4.809 g/cm³
- **Electronegativity:** 2.55 (Pauling)
- **Oxidation states:** +6, +4, -2
- **Discovered:** 1817 by Jöns Jacob Berzelius

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
- **Noble gas shorthand:** [Ar] 4s² 3d¹⁰ 4p⁴
- **Shells:** K: 2 · L: 8 · M: 18 · N: 6

## What it is used for

- Decolourising the green tint of iron in bottle glass
- Selenium sulfide in anti-dandruff shampoo
- Photoreceptor drums in older photocopiers and laser printers
- An essential trace nutrient, and an additive in livestock feed
- Pigments — the red in some traffic lights and warning glass

## Hazards

- The margin between the required intake and toxicity is small, roughly a factor of seven
- Hydrogen selenide is highly toxic and detectable by smell only above dangerous levels

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 32 | Germanium | Ge | 72.63 |
| 33 | Arsenic | As | 74.922 |
| 34 | Selenium ← | Se | 78.971 |
| 35 | Bromine | Br | 79.904 |
| 36 | Krypton | Kr | 83.798 |

## Group 16

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Selenium sits in group 16 with oxygen, sulfur, tellurium, polonium, livermorium.

## Common questions

### What is the atomic number of selenium?

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

### What is the symbol for selenium?

Se.

### What is the electron configuration of selenium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴, or [Ar] 4s² 3d¹⁰ 4p⁴ in noble-gas shorthand.

### What is the melting point of selenium?

493.65 K — 220.5 °C — 428.9 °F

## Related pages

- [Germanium (Ge)](https://atoneplace.net/element/germanium/)
- [Arsenic (As)](https://atoneplace.net/element/arsenic/)
- [Bromine (Br)](https://atoneplace.net/element/bromine/)
- [Krypton (Kr)](https://atoneplace.net/element/krypton/)
- [Oxygen (O)](https://atoneplace.net/element/oxygen/)
- [Sulfur (S)](https://atoneplace.net/element/sulfur/)
- [Tellurium (Te)](https://atoneplace.net/element/tellurium/)
- [Polonium (Po)](https://atoneplace.net/element/polonium/)
- [Livermorium (Lv)](https://atoneplace.net/element/livermorium/)

## Sources

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