All these elements are chemically stable by themselves. It is only in the presence of other elements that they are chemically reactive.
Atom Group Valence electrons Reactivity
1) Mg 2 2 Fairly reactive
2) Cs 1 1 Highly reactive
3) Ne 18 8 Completely unreactive
4) C 14 4 Relatively unreactive
5) Kr 18 8 Almost completely unreactive
Cs is the most reactive element because it needs to lose only one valence electron to get a stable octet.
Mg is less reactive than Cs because it must lose two valence electrons to get a stable octet.
C is relatively unreactive because it must either lose or gain four valence electrons to get a stable octet.
Ne is completely unreactive because it already has a stable octet.
Kr also has a stable octet, but its valence electrons are so far from the nucleus that some highly reactive elements will react with it.
