MetalloidElement 33 · p-block

Arsenic (As)

Arsenic (As) is a metalloid with atomic number 33 and a standard atomic weight of 74.922. At room temperature it is a solid that sublimes at 614 °C.

33AsArsenic74.922

Atomic number

protons

Atomic weight

74.922

CIAAW 2024

Melting point

—

Boiling point

614 °C

sublimes at 1 atm

Density

5.75 g/cm³

near room temperature

Electronegativity

2.18

Pauling scale

State at 25 °C

Solid

Discovered

c. 1250

Germany

Arsenic at a glance

Identity

Symbol	As
Atomic number (Z)	33
Atomic weight	74.922 ± 0.001 (CIAAW 2024)
Category	Metalloid
Group	15 — pnictogens (nitrogen group)
Period	4
Block	p-block
CAS number	7440-38-2

Atomic properties

Electron configuration	[Ar] 3d¹⁰ 4s² 4p³ Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³
Electrons per shell	2, 8, 18, 5
Oxidation states	-3, +3, +5 Less common: -2, -1, 0, +1, +2, +4
Electronegativity (Pauling)	2.18
First ionisation energy	9.789 eV (944.5 kJ/mol) 2nd: 18.589 eV · 3rd: 28.349 eV
Electron affinity	0.804 eV
Atomic radius (empirical)	115 pm
Covalent radius	121 pm
Van der Waals radius	185 pm

Physical properties

State at 25 °C	Solid
Boiling / sublimation point	613.9 °C · 1,136.9 °F · 887 K sublimes at 1 atm
Density	5.75 g/cm³
Specific heat capacity	0.329 J/(g·K)
Crystal structure	Rhombohedral

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	⁷⁵As 100%
Stable isotopes	5
Known isotopes	33
Abundance in Earth's crust	1.8 mg/kg
Abundance in seawater	0.0037 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Arsenic electron configuration

Arsenic has 33 electrons. Written in short form, its ground-state configuration is [Ar] 3d¹⁰ 4s² 4p³ — the electrons of the noble gas argon, plus 3d¹⁰ 4s² 4p³.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³.

As a member of group 15 it has 5 valence electrons in its outer shell (shell 4), which largely decide its chemistry.

3d↑↓↑↓↑↓↑↓↑↓

4s↑↓

4p↑↑↑

Discovery and name

Discovered

c. 1250 — Albertus Magnus (traditionally credited) · Germany.

Name

Greek arsenikon, yellow orpiment.

Known for

A notorious poison historically, but gallium arsenide is a key semiconductor and arsenic is used to harden lead alloys.

Isotopes of arsenic

Naturally occurring arsenic is a mix of 1 isotope. Each has 33 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
⁷⁵As	33	42	100%	stable

33 isotopes of arsenic are known in our data set (half-lives as published in nuclear data tables).

Arsenic compared with group 15

The other elements in group 15 (pnictogens (nitrogen group)), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Nitrogen	7	14.007	-210 °C	-196 °C	1.2506 g/L	3.04
Phosphorus	15	30.974	44 °C	281 °C	1.823 g/cm³	2.19
Arsenic	33	74.922	—	614 °C	5.75 g/cm³	2.18
Antimony	51	121.76	631 °C	1,587 °C	6.68 g/cm³	2.05
Bismuth	83	208.98	271 °C	1,564 °C	9.79 g/cm³	2.02
Moscovium	115	[290]	—	—	13.5 g/cm³ (pred.)	—

Frequently asked questions

6 answers

What is the atomic mass of arsenic?

The standard atomic weight of arsenic is 74.922 ± 0.001 (CIAAW 2024), so one mole weighs about 74.922 grams.

How many protons, neutrons and electrons does arsenic have?

A neutral arsenic atom has 33 protons and 33 electrons. Its most common isotope, arsenic-75, has 42 neutrons.

What is the electron configuration of arsenic?

[Ar] 3d¹⁰ 4s² 4p³ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³).

What are the melting and boiling points of arsenic?

Arsenic sublimes at 613.85 °C (1,136.9 °F).

Is arsenic a metal?

Arsenic is a metalloid — it has properties between metals and nonmetals. It is in group 15, period 4, and is a solid at room temperature.

Who discovered arsenic?

c. 1250 — Albertus Magnus (traditionally credited) (Germany).