What primarily determines the reactivity of an element in the periodic table?

The reactivity of an element is primarily determined by the number of valence electrons it possesses. Valence electrons are the outermost electrons in an atom and are crucial for chemical bonding. Elements tend to react in ways that allow them to achieve a full outer electron shell, which typically means gaining, losing, or sharing electrons with other atoms.

For instance, elements in Group 1 of the periodic table, like lithium and sodium, have one valence electron, making them highly reactive as they readily lose that electron to achieve a more stable electron configuration. Similarly, halogens have seven valence electrons and are very reactive because they seek to gain one more electron to complete their outer shell. This characteristic behavior explains why the number of valence electrons is a significant factor in determining reactivity.

While the atomic mass, position, and electronegativity of an element can influence its behavior, the core reason that drives elemental reactivity is fundamentally based on the electron configurations, specifically the valence electrons, making their count the primary determinant of reactivity in the periodic table.