The reactivity of any element depends on its valence electrons, its atomic radius and its electronegativity.
Explanation:
Valence electrons are electrons occupying the outer shell of the atom. Therefore they're responsible for all of chemical properties and some of the physical properties of the element. They determine the number of bonds the atom can form. For instance, the carbon atom has four valence electrons. Thus, any carbon atom can form four covalent bonds with surrounding atoms. On the other hand, hydrogen atoms, which have only one valence electron, can only form one covalent bond.
The atomic radius, defined as half the distance between the nuclei of two adjacent atoms, affects reactivity as well. The smaller the radius, the more reactive the element is. For example, it is possible compare the reactivity of Fluorine to the one of Iodine. The first is smaller than Iodine, which means that the electron's cloud allows the fluorine nucleus to attract more strongly the outer electrons: both its own and the electrons of other atoms.
These two previous properties combined give place to a third one: electronegativity. Which is a measure of the tendency ot the atoms in molecules to attract electrons to themselves.
In general, an element with small atoms has more electronegativity than a big one.
Additionally some elements having big atoms (and thus with low electronegativity) are said to be very reactive too because of their tendency to lose easily their valence electrons (i.e., Francium).
