Barium reacts quickly with cold water, while magnesium hardly reacts with it.

Group 1 metals are soft, have one outer electron and react with water to give hydrogen and an alkali; reactivity rises down the group. Group 2 metals are harder and less reactive. Halogens, group 17, get darker and less reactive down the group, and a more reactive halogen displaces a less reactive one. Noble gases have full shells.