Sodium at a glance
- Atomic number
- 11
- Symbol
- Na
- Atomic mass
- 22.99 u
- Category
- Alkali metal
- Group / period
- 1 / 3
- Electron configuration
- [Ne] 3s 1
- State at 25 °C
- Solid
- Melting point
- 97.72 °C
- Boiling point
- 882.9 °C
- Density
- 0.971 g/cm³
- Electronegativity
- 0.93
- 1st ionization energy
- 496 kJ/mol
- Oxidation states
- +1
- Discovered
- 1807
Atomic structure
A neutral sodium atom has 11 protons and 11 electrons, and its most common isotope carries about 12 neutrons. One atom weighs about 3.82 × 10⁻²⁶ kg, so a mole of sodium (6.022 × 10²³ atoms) weighs 22.99 g. That nucleus holds 1.09 neutrons for every proton. Its symbol Na comes from natrium. The electrons fill 3 shells (K 2, L 8, M 1), which gives the configuration [Ne] 3s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, Na-23. No other element has a stable isotope with mass number 23, so Na-23 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) sodium is a solid. It melts at 97.72 °C and boils at 882.9 °C, so it stays liquid across a 785.1-degree range. It stays solid in your hand (body temperature) but would melt in boiling water. Its density is 0.971 g/cm³, lighter than water. Its atoms have a radius of about 180 pm (Magnesium next door is 150 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.93 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 496 kJ/mol, less than Lithium above it (520 kJ/mol). A bond between sodium and chlorine would be mostly ionic, with an electronegativity difference of 2.2. It releases 53 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as Na⁺. Group 1 also holds Hydrogen, Lithium, Potassium, Rubidium, Caesium and Francium, which share its outer-shell pattern. Its Lewis symbol is Na surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 2.36 % of Earth's crust, the 6th most abundant element there. In the human body it accounts for about 0.2 % of body mass. It was first identified in 1807, making it the 42nd element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 589.0 nm, 589.6 nm and 568.8 nm. In a flame test its compounds colour the flame bright yellow-orange.