Atoms
of metals have less than four electrons in their valence shell.
When they react with other elements metal atoms lose their valence electrons
to attain a structure similar to that of a noble gas so that they
are stable.
The number of electrons metal atoms lose is equal
to the group number the metal belongs to.
Within
metals, the reactivity increases as you go down the group as the valence
electron(s) are held less and less strongly because they are found
farther and farther away from the attractive influence of the nucleus.
Density,
melting point, and boiling point also decreases going down group
1 and group 2 metals.