Boron at a glance
- Atomic number
- 5
- Symbol
- B
- Atomic mass
- 10.81 u
- Category
- Metalloid
- Group / period
- 13 / 2
- Electron configuration
- [He] 2s 2 2p 1
- State at 25 °C
- Solid
- Melting point
- 2076 °C
- Boiling point
- 3927 °C
- Density
- 2.34 g/cm³
- Electronegativity
- 2.04
- 1st ionization energy
- 801 kJ/mol
- Oxidation states
- +3
- Discovered
- 1808
Atomic structure
A neutral boron atom has 5 protons and 5 electrons, and its most common isotope carries about 6 neutrons. One atom weighs about 1.8 × 10⁻²⁶ kg, so a mole of boron (6.022 × 10²³ atoms) weighs 10.81 g. That nucleus holds 1.2 neutrons for every proton. The electrons fill 2 shells (K 2, L 3), which gives the configuration [He] 2s 2 2p 1. With 3 electrons in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: B-10 and B-11. No other element has a stable isotope with mass number 11, so B-11 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) boron is a solid. It melts at 2076 °C and boils at 3927 °C, so it stays liquid across a 1851-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 12th highest melting point of any element. Its density is 2.34 g/cm³, 2.34 times as dense as water. Its atoms have a radius of about 85 pm (Beryllium next door is 105 pm). In the solid, its atoms pack into a rhombohedral lattice.
Chemical behaviour
Its electronegativity is 2.04 on the Pauling scale. Pulling off its outermost electron takes 801 kJ/mol. A bond between boron and chlorine would be polar covalent, with an electronegativity difference of 1.1. It releases 27 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as B³⁺. Group 13 also holds Aluminium, Gallium, Indium, Thallium and Nihonium, which share its outer-shell pattern. Its Lewis symbol is B surrounded by 3 dots.
Where it comes from
It makes up 10 mg/kg of Earth's crust, the 37th most abundant element there. It was first identified in 1808, making it the 44th element to be discovered (counting the 11 known since antiquity together). In a flame test its compounds colour the flame bright green.