---
title: "Selenium (Se): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/selenium.html
description: "Selenium (Se), element 34: atomic mass 78.971, electron configuration [Ar] 3d¹⁰ 4s² 4p⁴, group 16, period 4, p-block reactive nonmetal. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
34Se78.971

# Selenium (Se) – element 34

Selenium is a chemical element with the symbol **Se** and atomic number **34**. It is a reactive nonmetal in group 16 (chalcogens), period 4 of the periodic table (p-block), and is a solid at room temperature.

Selenium (Se) has atomic number 34, atomic mass 78.971 u and electron configuration [Ar] 3d10 4s2 4p4.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Selenium facts

| Symbol | Se |
| --- | --- |
| Atomic number | 34 |
| Atomic mass | 78.971 u (g/mol) |
| Category | reactive nonmetal |
| Group | group 16 (chalcogens) |
| Period | 4 |
| Block | p-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 |
| Noble-gas notation | [Ar] 3d10 4s2 4p4 |
| Electrons per shell | 2, 8, 18, 6 |
| Valence electrons | 6 |

## Bohr model and shells

Selenium has 34 electrons in 4 shells: 2, 8, 18, 6. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 494 K (220.9 °C) |
| --- | --- |
| Boiling point | 958 K (684.9 °C) |
| Density | 4.81 g/cm³ |
| Electronegativity (Pauling) | 2.55 |
| Electron affinity | 194.959 kJ/mol |
| Ionization energies | 941, 2045, 2973.7 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 25.363 J/(mol·K) |
| Discovered by | Jöns Jakob Berzelius |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing Se.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of selenium?

The atomic mass of selenium is 78.971 u, so one mole of selenium atoms weighs 78.971 g.

### What is the electron configuration of selenium?

Selenium has 34 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴, or [Ar] 3d¹⁰ 4s² 4p⁴ in noble-gas notation. Electrons per shell: 2, 8, 18, 6.

### How many valence electrons does selenium have?

Selenium has 6 valence electrons (group 16).

### Is selenium a metal?

No. Selenium is a nonmetal (reactive nonmetal).

### Where is selenium on the periodic table?

Selenium is in group 16 (chalcogens), period 4, in the p-block.

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.