Final answer:
Valence electrons are crucial for chemical reactivity as they are the ones that participate in bonding, seeking to achieve filled valence shells for stability, unlike the inner-shell electrons.
Step-by-step explanation:
The number of valence electrons is more important than the number of total electrons because it is these outermost electrons that determine an atom's chemical behavior. Valence electrons are involved in forming chemical bonds with other atoms. Atoms strive to reach a state of stability by filling their valence shell, which often consists of eight electrons, known as the octet rule. For example, hydrogen has one valence electron and seeks stability by fulfilling its valence shell through bonding. The number of valence electrons increases as you move from left to right across a period in the periodic table, determining the reactivity of the elements.
An atom that does not have a full valence shell is more likely to be reactive. To achieve a full valence shell, an atom may lose, gain, or share electrons with other atoms, thus creating ions or covalent bonds. Atoms with a full valence shell, like the noble gases, are inert because their valence shell is filled and thus they do not tend to engage in chemical reactions easily.