A potassium atom has 19 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell, 8 in the M shell and 1 in the N shell.
The M shell can hold up to 18 electrons, but in potassium, once the M shell has 8, the next electron goes into the N shell.
The single electron in the outermost N shell is the valence electron, and by giving it up, potassium becomes a singly charged positive ion, \mathrm{K}^{+}. Because this outer electron is farther from the nucleus than in sodium, potassium gives it up even more readily than sodium does.
When potassium is put into water, it reacts even more violently than sodium. The heat of the reaction sets fire to the hydrogen given off, and the potassium moves quickly about on the surface with a lilac flame.
2\mathrm{K} + 2\mathrm{H_2O} \longrightarrow 2\mathrm{KOH} + \mathrm{H_2}
The potassium hydroxide formed is a strong base, so water containing phenolphthalein turns pink.
Potassium also reacts quickly with oxygen and moisture in the air, so, like sodium, it is stored under kerosene.
When a platinum wire dipped in a solution containing potassium is held in the flame of a gas burner, the flame turns lilac. Potassium gives out red light with a wavelength of about 766 nm and violet light of about 404 nm, and together they look lilac.
If sodium is also present, its strong yellow light makes the lilac hard to see. In that case, the flame is viewed through blue cobalt glass, which does not let the yellow light through.
Potassium is one of the three main nutrients in fertilizers, along with nitrogen and phosphorus. Plants take up potassium ions through their roots and use them, for example, to control the amount of water in their tissues and to carry the sugars made by photosynthesis.
The ash left after burning plants is rich in potassium carbonate, and it has been used as a fertilizer since ancient times. The English name potassium comes from potash, the ash of plants, and the symbol K comes from the Latin name kalium, which is taken from the Arabic word for plant ash.
Inside the cells of our bodies there are many potassium ions, \mathrm{K}^{+}, while outside the cells there are many sodium ions, \mathrm{Na}^{+}.
A pump in the cell membrane keeps up this difference: each time it moves three sodium ions out, it brings two potassium ions in. Nerves use this difference in ions to send signals, and the muscles of the heart use it to contract.
Potassium is plentiful in vegetables, fruit and potatoes.