Arsenic

Arsenic (As) is a metalloid with atomic number 33 and an atomic mass of 74.922. Its electron configuration is [Ar] 3d10 4s2 4p3, which gives it 5 valence electrons.

Arsenic key data

Symbol	As
Atomic number	33
Atomic mass (standard atomic weight)	74.922
Molar mass	74.922 g/mol
Protons	33
Neutrons (most abundant isotope, arsenic-75)	42
Electrons (neutral atom)	33
Valence electrons	5
Category	Metalloid
Period	4
Group	15
Block	p-block
Noble gas notation	[Ar] 3d10 4s2 4p3
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3
Electrons per shell	2, 8, 18, 5

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of arsenic

From PubChem, the US National Institutes of Health. Public domain.

Melting point (only under pressure)	1090 K (816.85 °C), 60th of 103 by melting point
Sublimation point	887 K (613.85 °C), 77th of 93 by boiling point
Density	5.776 g/cm³, 60th of 96 by density
State at room temperature	Solid
Electronegativity	2.18 (Pauling), 24th of 95 by electronegativity
Van der Waals radius	185 pm, 77th of 99 by atomic radius
First ionisation energy	9.815 eV (947 kJ/mol), 18th of 102 by first ionisation energy
Electron affinity	0.81 eV (78.2 kJ/mol), 28th of 57 by electron affinity
Common oxidation states	+5, +3, −3
Discovered	Ancient

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Arsenic electron configuration

The full electron configuration of arsenic is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3. With the noble-gas core collapsed it is [Ar] 3d10 4s2 4p3, and the outermost subshell is 4p3.

Bohr model of arsenic (As): a nucleus and four shells holding 2, 8, 18 and 5 electrons from the inside out. — Bohr model of arsenic (As). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of arsenic (As), 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3: one box per orbital, filled by Hund’s rule, showing three unpaired electrons, in 4p. — Orbital diagram of arsenic (As). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one arsenic atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 74.922 / (6.02214076 × 10²³)
= 1.2441 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 74.922
= 8.0379 × 10²¹ atoms

Arsenic on the periodic table

Arsenic is in period 4, group 15, in the p-block.

The other elements of group 15 are nitrogen, phosphorus, antimony, bismuth and moscovium.