Silicon at a glance
- Atomic number
- 14
- Symbol
- Si
- Atomic mass
- 28.085 u
- Category
- Metalloid
- Group / period
- 14 / 3
- Electron configuration
- [Ne] 3s 2 3p 2
- State at 25 °C
- Solid
- Melting point
- 1414 °C
- Boiling point
- 3265 °C
- Density
- 2.33 g/cm³
- Electronegativity
- 1.9
- 1st ionization energy
- 787 kJ/mol
- Oxidation states
- +4, −4
- Discovered
- 1824
Atomic structure
A neutral silicon atom has 14 protons and 14 electrons, and its most common isotope carries about 14 neutrons. One atom weighs about 4.66 × 10⁻²⁶ kg, so a mole of silicon (6.022 × 10²³ atoms) weighs 28.085 g. That nucleus holds 1 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 3 shells (K 2, L 8, M 4), which gives the configuration [Ne] 3s 2 3p 2. With 4 electrons in its outer shell, it usually shares electrons in covalent bonds. It has 3 stable isotopes: Si-28, Si-29 and Si-30. No other element has a stable isotope with mass number 28, so Si-28 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) silicon is a solid. It melts at 1414 °C and boils at 3265 °C, so it stays liquid across a 1851-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 2.33 g/cm³, 2.33 times as dense as water. Its atoms have a radius of about 110 pm (Aluminium next door is 125 pm). In the solid, its atoms pack into a diamond cubic lattice.
Chemical behaviour
Its electronegativity is 1.9 on the Pauling scale. Pulling off its outermost electron takes 787 kJ/mol, less than Carbon above it (1086 kJ/mol). A bond between silicon and chlorine would be polar covalent, with an electronegativity difference of 1.3. It releases 134 kJ/mol when it gains an extra electron. Its common oxidation states are +4, −4, so in compounds you will usually meet it as Si⁴⁺ and Si⁴⁻. Group 14 also holds Carbon, Germanium, Tin, Lead and Flerovium, which share its outer-shell pattern. Its Lewis symbol is Si surrounded by 4 dots.
Where it comes from
It makes up 28.2 % of Earth's crust, the 2nd most abundant element there. In the human body it accounts for about 0.002 % of body mass. It was first identified in 1824, making it the 51st element to be discovered (counting the 11 known since antiquity together).