Arsenic at a glance
- Atomic number
- 33
- Symbol
- As
- Atomic mass
- 74.922 u
- Category
- Metalloid
- Group / period
- 15 / 4
- Electron configuration
- [Ar] 3d 10 4s 2 4p 3
- State at 25 °C
- Solid
- Melting point
- 816.9 °C
- Boiling point
- 613.9 °C
- Density
- 5.776 g/cm³
- Electronegativity
- 2.18
- 1st ionization energy
- 947 kJ/mol
- Oxidation states
- +3, +5, −3
- Discovered
- 1250
Atomic structure
A neutral arsenic atom has 33 protons and 33 electrons, and its most common isotope carries about 42 neutrons. One atom weighs about 1.24 × 10⁻²⁵ kg, so a mole of arsenic (6.022 × 10²³ atoms) weighs 74.922 g. That nucleus holds 1.27 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 5), which gives the configuration [Ar] 3d 10 4s 2 4p 3. With 5 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has only one stable isotope, As-75. No other element has a stable isotope with mass number 75, so As-75 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) arsenic is a solid. It melts at 816.9 °C and boils at 613.9 °C, so it stays liquid across a -203-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 5.776 g/cm³, 5.78 times as dense as water. Its atoms have a radius of about 115 pm (Germanium next door is 125 pm). In the solid, its atoms pack into a rhombohedral lattice.
Chemical behaviour
Its electronegativity is 2.18 on the Pauling scale. Pulling off its outermost electron takes 947 kJ/mol, less than Phosphorus above it (1012 kJ/mol). A bond between arsenic and chlorine would be polar covalent, with an electronegativity difference of 0.98. It releases 79 kJ/mol when it gains an extra electron. Its common oxidation states are +3, +5, −3, so in compounds you will usually meet it as As³⁺, As⁵⁺ and As³⁻. Group 15 also holds Nitrogen, Phosphorus, Antimony, Bismuth and Moscovium, which share its outer-shell pattern. Its Lewis symbol is As surrounded by 5 dots. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 1.8 mg/kg of Earth's crust, the 54th most abundant element there. It was first identified in 1250, making it the 12th element to be discovered (counting the 11 known since antiquity together).