The First Main Group - Alkali Metals

The name 'alkali metals' is derived from the Arabic word "al kali", which is the term for the potassium carbonate found in plant ashes. Potassium carbonate forms an alkaline solution when it is mixed with water.

The elements of this group are hydrogen, lithium, sodium, potassium, rubidium, caesium and francium. All of them, except for hydrogen, react intensely with water by forming hydrogen and an alkaline solution. They are relatively soft and conduct electricity.

Lithium, potassium and sodium are the elements with the least density. Caesium and rubidium are the ones with the least temper. Alkali metals only possess a single valence electron in the outer electron shell, which explains their high reactivity, as this valence electron can be easily lost.

Sodium: Pure sodium is a silvery alkali metal, which immediately turns grey when it is exposed to moist air since a sodium hydroxide layer is formed on its surface. In the open air, sodium binds oxygen, water and carbon dioxide and turns into a mixture of sodium hydroxide, sodium carbonate and sodium hydrogen carbonate. For this reason it is stored in paraffin oil or petroleum for protection. Sodium is a very soft metal (according to Mohs’ scale its temper is 0.4) and can be easily cut with a knife.

Sodium vapour lamps used for lighting pedestrian crossings and streets contain solid sodium in addition to noble gases. The sodium evaporates as soon as the lamp is switched on and emits a yellow light. The human eye perceives this yellow light as particularly bright.

Potassium: Pure potassium is a very soft light metal with a silvery lustre. In the open air it becomes tarnished very quickly, forming a dull grey coating of potassium oxide, which slowly turns to potassium hydroxide and potassium carbonate in the end. Because of this characteristic, potassium is also stored in paraffin oil.

Caesium: The honey-coloured caesium is the softest element of all. It can be melted with the bare hands and has the lowest melting point of all elements except for mercury. Caesium can be easily ionised with UV irradiation and is therefore ideal for manufacturing photosensitive cells for electronics.