chem QQ

Down Group I or Group II, the reactivity of element increases. Down the group, the number of shells is increasing, the distance between the nucleus and the outermost shell is thus increasing, and thus the electrostatic attraction between the nucleus and the electron(s) in the outermost shell is decreasing. Therefore, the electron(s) in the outermost shell is/are lost more readily to give a cation.
Note that the reactivity of element decreases down Group VII. Besides, elements from Group III to Group VI do not show apparent trend in reactivity because each of these groups contains both metals and non-metals.

Down a group, the size of the element is bigger becoz it have more electronic shell, so distance between the outermost eletrons of the element and the proton is further away, so the attractional force between them is weaker. So it is more easy for that element to react with others.

2007-03-13 22:58:26 補充：
Remarks: the reactivity of an element denpends on the outermost electrons.