What determines how reactive an element is?
Well, its electronic structure............
You know that chemists have gone to great lengths in proposing and describing schemes to explain the electronic structure of a given element. You probably have already been introduced to these schemes, at SOME degree of complexity; now the electronic scheme does rationalize to an extent elemental reactivity.
Metals are electron-rich materials, they tend to be reducing, and this they commonly form
An incompletely filled valence shell of electrons shields the nuclear charge VERY ineffectively. The macroscopic result? As we proceed across a Period, an horizontal row of the Periodic Table, atomic radii TEND to decrease dramatically. When a Period is filled (i.e. at the Noble Gas end), electronic shielding of the nuclear charge becomes effective. And the next element, an alkali metal, begins to fill a new valence FARTHER removed from the nuclear core, and the
And metals, especially alkali, and alkaline earth metals, with the one or two valence electrons are prone to oxidation. On the other hand, and on the other end of the Periodic Table, oxygen and fluorine, with 6 and 7 valence electrons, want to fill their valence shell of electrons. Fluorine in particular is regarded as the most reactive element on the Periodic Table because of its oxidizing ability.