Why are atoms of metals and non metals reactive?

They bound using ionic bounding, the metal gives away electrons and the non-metal receive, and since electrons are negative in charge the metal becomes positive and the non-metal negative this makes them attract each other, that's what ionic compounds are mostly hard. They bound because the "aim" for an atom is to get an full outer shell. the group number will tell you how many outer electrons it has (don't get fooled by some periodic tables where all the rows are labeled it is actually just the rows which "stands up" you can see this in the other will normally say it next to themex: Fe(iii)= three outer electrons.) and the outer shell normally want 8 electrons.

ex:

Li F

Group 1 Group 7

Li+ F-