A sodium atom has 11 electrons around its nucleus. The electrons fill the shells from the inside outwards: 2 in the K shell, 8 in the L shell and 1 in the M shell.
The outermost M shell holds just one electron. This electron is called the valence electron, and much of the behaviour of sodium is decided by it.
Lithium, potassium and the other alkali metals also have a single electron in their outermost shell.
A sodium atom readily gives up its single outer electron. After losing it, the atom has 10 electrons left, arranged in the same way as those of a neon atom.
With one electron fewer, the positive charge of the nucleus is greater than the total charge of the electrons by one. The result is the sodium ion, \mathrm{Na}^{+}, a singly charged positive ion.
Because the M shell is gone, the ion is much smaller than the atom. The radius of the atom is about 186 pm and that of the ion is about 102 pm (1 pm is one trillionth of a metre).
In solid sodium, sodium ions are arranged in a regular pattern, and the valence electrons move freely among them. These electrons are called free electrons, and the bonding of the ions to one another through the free electrons is called metallic bonding.
Each sodium atom gives only one free electron, so the metallic bonding in sodium is weak. As a result, sodium is soft enough to cut with a knife, and it melts at a low temperature of about 98 °C.
A freshly cut surface is a shiny silvery white, but it soon turns dull as it reacts with oxygen and water vapour in the air.
When sodium is put into water, it reacts vigorously and gives off hydrogen. Sodium is less dense than water, so it floats; the heat of the reaction melts it into a ball, which moves quickly about on the surface.
The sodium hydroxide formed in the reaction is a strong base, so water containing phenolphthalein turns pink.
2\mathrm{Na} + 2\mathrm{H_2O} \longrightarrow 2\mathrm{NaOH} + \mathrm{H_2}
Sodium also reacts with moisture in the air, so it is stored under kerosene.
When a platinum wire dipped in a solution containing sodium is held in the flame of a gas burner, the flame turns a bright yellow. Each element that colours a flame gives it its own colour, and this is the basis of the flame test.
The heat of the flame raises the outer electron to a state of higher energy, and when the electron falls back, it gives out light of a particular wavelength. For sodium, this is yellow light with a wavelength of about 589 nm.
When salty soup boils over onto a gas stove, the flame turns yellow for the same reason: the sodium in the salt.