Aluminium at a glance
- Atomic number
- 13
- Symbol
- Al
- Atomic mass
- 26.982 u
- Category
- Post-transition metal
- Group / period
- 13 / 3
- Electron configuration
- [Ne] 3s 2 3p 1
- State at 25 °C
- Solid
- Melting point
- 660.3 °C
- Boiling point
- 2519 °C
- Density
- 2.698 g/cm³
- Electronegativity
- 1.61
- 1st ionization energy
- 578 kJ/mol
- Oxidation states
- +3
- Discovered
- 1825
Atomic structure
A neutral aluminium atom has 13 protons and 13 electrons, and its most common isotope carries about 14 neutrons. One atom weighs about 4.48 × 10⁻²⁶ kg, so a mole of aluminium (6.022 × 10²³ atoms) weighs 26.982 g. That nucleus holds 1.08 neutrons for every proton. The electrons fill 3 shells (K 2, L 8, M 3), which gives the configuration [Ne] 3s 2 3p 1. With 3 electrons in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, Al-27. No other element has a stable isotope with mass number 27, so Al-27 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) aluminium is a solid. It melts at 660.3 °C and boils at 2519 °C, so it stays liquid across a 1859-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 2.698 g/cm³, 2.7 times as dense as water. Its atoms have a radius of about 125 pm (Magnesium next door is 150 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.61 on the Pauling scale. Pulling off its outermost electron takes 578 kJ/mol, less than Boron above it (801 kJ/mol). A bond between aluminium and chlorine would be polar covalent, with an electronegativity difference of 1.6. It releases 42 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as Al³⁺. Group 13 also holds Boron, Gallium, Indium, Thallium and Nihonium, which share its outer-shell pattern. Its Lewis symbol is Al surrounded by 3 dots.
Where it comes from
It makes up 8.23 % of Earth's crust, the 3rd most abundant element there. In the human body it accounts for about 0.00009 % of body mass. It was first identified in 1825, making it the 52nd element to be discovered (counting the 11 known since antiquity together).