The elements in Group 1 are known as the alkali metals. Their characteristic properties come from the single electron in their outer shell. Each atom loses this one electron when it reacts and forms a positive ion. Group 1 metals are soft and are stored under oil, and lithium, sodium and potassium are less dense than water.
The first three alkali metals (lithium, sodium and potassium) react with oxygen to form metal oxides, for example 4Li + O2 → 2Li2O. They react with chlorine to form white metal chlorides, for example 2Na + Cl2 → 2NaCl.
With water they form a metal hydroxide and hydrogen, for example 2Na + 2H2O → 2NaOH + H2. The hydroxide dissolves to give an alkaline solution. Lithium fizzes steadily, sodium melts into a ball and fizzes quickly, and potassium also gives a lilac flame.
Reactivity increases going down Group 1. Further down, the outer electron is further from the nucleus and is held less strongly, so it is lost more easily. This lets you predict that potassium is more reactive than sodium, and that rubidium is more reactive still.
