The Periodic table!
AND NOW.....
FIRST, THE STAR OF THE SHOW!!!
More to it than you think!
SHELLS?
GROUPS!!!
Group 1, which are the alkali metals are very reactive with water as it only has 1 valence electron, which makes it easily bondable. These metals are easily cut with a knife. The first 3 metals (Litium, potassium and Sodium) are the only metals that are allowed in schools out of the group!
Group 7, Which are called HALOGENS, Have 7 valence electrons, These Elements react very easily with Group 1. The lower you go down this group the less reactive the elements are. The Elements have different states of matter at room temperature, first two being a gas and the third a liquid.
Group 8 AKA the noble gases, is the last group and is least reactive out of all the 8 because it has 8 valence electrons not making it very "bondable" with any other element. They are all gases and do not form compounds.