ElementsWhat is arsenic?
Arsenic is a grey, brittle element known since ancient times. It and many of its compounds are very poisonous. In some regions it occurs naturally in groundwater, which is a serious health problem.
What is arsenic used for?
Hardening the lead in car batteries, and gallium arsenide chips. Older uses in pesticides and wood preservatives have been restricted.
Where the name comes from
From the Greek word arsenikon, the name of a yellow mineral that contains it.
Key facts
- Symbol
- As
- Atomic number
- 33
- Atomic mass
- 74.92159 u
- Family
- Metalloid
- Period
- 4
- Group
- 15
- Block
- p
- State at room temperature
- Solid
- Melting point
- 816.9 °C (1,090 K, 1,502 °F)
- Boiling point
- 613.9 °C (887 K, 1,137 °F)
- Density
- 5.78 g/cm³
- Electrons per shell
- 2, 8, 18, 5
- Electron configuration
- [Ar]4s2 3d10 4p3
- Common oxidation states
- +5, +3, -3
- Electronegativity (Pauling)
- 2.18
- First ionisation energy
- 9.815 eV
- Electron affinity
- 0.81 eV
- Atomic radius (van der Waals)
- 185 pm
- Discovered
- Known since ancient times
Place in the periodic table
Arsenic is element number 33. It sits in period 4 (row 4) of the periodic table, in group 15 (column 15), and belongs to the p-block. Just before it comes germanium (32), and just after it comes selenium (34). In the same column, phosphorus is above it and antimony is below it. Elements in one column tend to behave alike.
Family: Metalloids.
Physical properties
At room temperature arsenic is a solid. At normal pressure it does not melt when heated: it turns straight from solid to vapour at about 613.9 °C. The melting point given, 816.9 °C, is measured under high pressure. Its density is 5.78 grams per cubic centimetre, so a one-litre block would weigh about 5.78 kilograms. By density it ranks 60th among the 96 elements with a measured value.
Inside the atom
An atom of arsenic has 33 protons in its nucleus and, when neutral, 33 electrons around it. The electrons are arranged in 4 shells: 2, 8, 18, 5. The outermost shell holds 5 electrons. Written in full, the electron configuration is [Ar]4s2 3d10 4p3.
Its electronegativity on the Pauling scale is 2.18. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 9.815 electronvolts (about 947 kilojoules per mole). One mole of arsenic weighs 74.92159 grams, so a single gram contains about 8.04 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains arsenic with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.