A metal atom's aptitude to lose electrons to other atoms causes it to be more reactive. Involved in the scientifically determined aptitude is the speed at which a metal atom can lose electrons, as well as the substances with which the atom is likely to react.
Metals belong to one of four classes depending on their reactivity potential, with the first class containing the most reactive metals. When metal atoms react with and lose electrons to water, oxygen or an acid, it tarnishes or corrodes, producing positive and negative ions. Metals that react with water or oxygen, found within the first class of reactive metals, are more reactive than those that need an acid to undergo a chemical reaction.
The strength of the acid needed by a metal to produce a chemical reaction determines its level of reactivity. For example, copper demands an acid strong enough to oxidize its atoms and therefore resides in the third class. On the other hand, aluminum, which rapidly reacts with less potent acids, belongs to the second class.
One way to predict a metal's ability to react is by examining the electron structure of its atoms compared to the electron structure of its reactant. The valence electrons, those found in the outermost shell of an atom, determine the atom's degree of electronegativity. Metal atoms with lower electronegativity lose valence electrons more easily and consequentially qualify as more reactive. These atoms are capable of greater reactivity because they combine readily with the atoms in water and oxygen, which possess high degrees of electronegativity.