All alkali metals are very reactive, therefore they have to be stored in oil which prevents them froom reacting with the oxygen in the air. Their reactivity increases as you go down in the group.
All alkali metals have very low density compared with other metals, which means they can float and that they are very soft and can be cut with a knife. They have a very shiny, silvery surface when you first cut then, howver this quickly goes dull as the metals react with the oxygen in the air. This forms a layer of oxide on the surface for example:
Sodium + Oxygen = Sodium Oxide
4Na(s) + O2(g) = 2Na2O
In a gas jar of oxygen, hot alkali metals burn vigorously, forming a white smoke of their oxides.
The properties of this unusual group is due to their electronic structure. Alkali metal atoms have only one electron in their outer-most shell. This makes them very reactive since they only need to lose one atom to get the stabe electronic structure of a noble gas. They form a metal ion with a +1 charge. They always form ionic compounds.
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