Arsenic (As)
Element 33 · Metalloid · atomic mass 74.922 u · group 15, period 4
Electron shells: 2, 8, 18, 5
Arsenic is the classic poison of history, disrupting cellular energy production at low doses. The same toxicity makes some of its compounds effective cancer drugs.
Arsenic trioxide went from Victorian murder weapon to a frontline leukaemia therapy.
- Atomic number
- 33
- Atomic mass
- 74.922 u
- Electron configuration
- [Ar] 4s² 3d¹⁰ 4p³
- Group · period · block
- 15 · 4 · p
- Oxidation states
- −3, +3, +5
- Electronegativity
- 2.18 (Pauling)
- Melting point
- 817 °C
- Boiling point
- 614 °C
- Density
- 5.776 g/cm³
- State at 20 °C
- Solid
- Appearance
- Metallic grey, brittle solid
- Discovered
- Albertus Magnus, 1250
33 protons, and 33 electrons when neutral
IUPAC standard atomic weight
Pnictogens
What Arsenic is used for
- Semiconductor doping and gallium arsenide
- Lead alloys for car batteries
- Leukaemia treatment
- Historically, wood preservatives
Atomic structure of Arsenic
A neutral arsenic atom has 33 protons and 33 electrons. Its most common isotope carries about 42 neutrons, which is where the mass number 75 comes from. Change the neutron count and you have a different isotope of arsenic; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 5. In orbital notation that is [Ar] 4s² 3d¹⁰ 4p³, leaving 5 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Arsenic's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Arsenic sits in the periodic table
Elements on the staircase between metals and non-metals, showing properties of both. Their intermediate conductivity is what makes semiconductors possible.
- Directly above: Phosphorus — same group, so similar chemistry with a smaller atom.
- Directly below: Antimony — same group, one shell larger.
- One proton lighter: Germanium.
- One proton heavier: Selenium.
Discovery and naming
Arsenic was discovered by Albertus Magnus in 1250. Its name comes from Greek arsenikon, a yellow pigment, which is also why its symbol (As) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Arsenic?
The standard atomic weight of Arsenic is 74.922 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Arsenic have?
Arsenic has 33 protons, because the atomic number is the proton count and that is what makes an atom Arsenic at all. A neutral Arsenic atom has the same number of electrons, 33. The neutron count varies by isotope; the most common one has about 42, since 75 − 33 = 42.
What is the electron configuration of Arsenic?
The electron configuration of Arsenic is [Ar] 4s² 3d¹⁰ 4p³. Its electrons are arranged 2, 8, 18, 5 across 4 shells.
Is Arsenic a metal?
Not quite. Arsenic is a metalloid — it sits on the staircase dividing metals from non-metals and shows properties of both, which is exactly why metalloids matter to the semiconductor industry.
What group and period is Arsenic in?
Arsenic is in group 15 (the pnictogens), period 4, in the p-block. The group is its column and the period is its row.
What is the charge of Arsenic?
Arsenic most commonly forms a 3− ion. It is 3 electrons short of a full outer shell, so it gains them rather than losing 5. Across all its compounds it shows oxidation states of −3, +3, +5.
What is the melting point of Arsenic?
Arsenic melts at 817 °C and boils at 614 °C. That puts it in the solids at everyday temperatures.
What is Arsenic used for?
Semiconductor doping and gallium arsenide, Lead alloys for car batteries, Leukaemia treatment, Historically, wood preservatives. Arsenic is the classic poison of history, disrupting cellular energy production at low doses.
Who discovered Arsenic, and where does its name come from?
Arsenic (As) was discovered by Albertus Magnus in 1250. The name comes from Greek arsenikon, a yellow pigment.