Reactivity refers to how readily a substance undergoes a chemical reaction with other substances. This property is crucial in chemistry, as it determines how elements and compounds interact. For example, sodium is highly reactive with water, producing heat and hydrogen gas, while gold is much less reactive and does not easily combine with other elements. The reactivity of an element is influenced by its atomic structure, particularly the arrangement of electrons. Elements in the same group of the periodic table often exhibit similar reactivity. For instance, the alkali metals are known for their high reactivity, while the noble gases are largely unreactive due to their full electron shells.