Arsenic (As)
Metalloid
Arsenic is the most notorious poison of history — yet this metalloid also hardens lead alloys, dopes semiconductors, and colours fireworks. Its toxicity comes from mimicking phosphorus in the body's chemistry.
Position on the periodic table
Atomic properties
| Atomic number | 33 |
|---|---|
| Atomic mass | 74.9216 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3 |
| Noble gas shorthand | [Ar] 3d10 4s2 4p3 |
| Electrons per shell | 2, 8, 18, 5 |
| Valence electrons (outer shell) | 5 |
| Common oxidation states | -3, +3, +5 |
| Electronegativity (Pauling) | 2.18 |
| Covalent radius (approx.) | 119 pm |
| First ionization energy | 947 kJ/mol |
| Electron affinity | 77.7 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 5.727 g/cm³ |
| Melting point | Not available |
| Boiling point | Not available |
| Appearance | metallic grey |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of arsenic
Arsenic's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3, usually shortened to [Ar] 3d10 4s2 4p3. Its electrons occupy 4 shells (2, 8, 18, 5), placing it in period 4 of the p-block. The 5 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of arsenic
- Gallium arsenide semiconductors
- Wood preservatives (declining use)
- Lead alloy hardener (car batteries, ammunition)
- Historic pigments and medicines (now obsolete)
Biological role: Possibly needed in ultratrace amounts by some animals; toxic to humans in tiny doses.
Occurrence: Found in ores such as arsenopyrite and as an impurity in many metal ores.
Common compounds of arsenic
As2O3
Arsenic trioxide
GaAs
Gallium arsenide
History and discovery
Discovered: 1250 — Bronze Age. Name origin: From Greek arsenikon, the name of its yellow mineral orpiment.
Arsenic compounds were known to ancient physicians; Albertus Magnus is credited with isolating the element around 1250.
Safety notes
Arsenic and its compounds are highly toxic and carcinogenic. Contaminated groundwater is a serious world health issue.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Arsenic
1. What is the chemical symbol of Arsenic?
2. What is the atomic number of Arsenic?
3. Which category does Arsenic belong to?
4. What is the state of Arsenic at room temperature?
5. Which period of the periodic table is Arsenic in?
Arsenic FAQs
What is the atomic number of arsenic?
Arsenic's atomic number is 33 — every arsenic atom has 33 protons in its nucleus.
What is the symbol for arsenic?
As. From Greek arsenikon, the name of its yellow mineral orpiment.
Is arsenic a metal, nonmetal, or metalloid?
Arsenic is classified as a metalloid — an element with properties between metals and nonmetals.
What state is arsenic at room temperature?
At about 25 °C, arsenic is a solid.
How many valence electrons does arsenic have?
Arsenic has 5 electrons in its outer shell (shell pattern: 2, 8, 18, 5).
What is arsenic used for?
Key uses include: gallium arsenide semiconductors; wood preservatives (declining use); lead alloy hardener (car batteries, ammunition).
Related elements
Keep working with this element
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