Beryllium at a glance
- Atomic number
- 4
- Symbol
- Be
- Atomic mass
- 9.0122 u
- Category
- Alkaline earth
- Group / period
- 2 / 2
- Electron configuration
- [He] 2s 2
- State at 25 °C
- Solid
- Melting point
- 1287 °C
- Boiling point
- 2469 °C
- Density
- 1.85 g/cm³
- Electronegativity
- 1.57
- 1st ionization energy
- 900 kJ/mol
- Oxidation states
- +2
- Discovered
- 1798
Atomic structure
A neutral beryllium atom has 4 protons and 4 electrons, and its most common isotope carries about 5 neutrons. One atom weighs about 1.5 × 10⁻²⁶ kg, so a mole of beryllium (6.022 × 10²³ atoms) weighs 9.0122 g. That nucleus holds 1.25 neutrons for every proton. The electrons fill 2 shells (K 2, L 2), which gives the configuration [He] 2s 2. With 2 electrons in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, Be-9. No other element has a stable isotope with mass number 9, so Be-9 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) beryllium is a solid. It melts at 1287 °C and boils at 2469 °C, so it stays liquid across a 1182-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 1.85 g/cm³, 1.85 times as dense as water. Its atoms have a radius of about 105 pm (Lithium next door is 145 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.57 on the Pauling scale. Pulling off its outermost electron takes 900 kJ/mol. A bond between beryllium and chlorine would be polar covalent, with an electronegativity difference of 1.6. Its common oxidation states are +2, so in compounds you will usually meet it as Be²⁺. Group 2 also holds Magnesium, Calcium, Strontium, Barium and Radium, which share its outer-shell pattern. Its Lewis symbol is Be surrounded by 2 dots. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 2.8 mg/kg of Earth's crust, the 48th most abundant element there. It was first identified in 1798, making it the 33rd element to be discovered (counting the 11 known since antiquity together).