Final answer:
Chemical reactivity is closely related to the number and arrangement of an atom's outermost valence electrons, where incomplete shells increase reactivity.
Step-by-step explanation:
There is a strong connection between chemical reactivity and the outermost valence electrons in an atom. Chemical reactivity is influenced by the arrangement and number of valence electrons. An atom with a full valence shell is generally stable and unreactive, while atoms with incomplete valence shells tend to be more reactive. For example, hydrogen with only one electron will be highly reactive, aiming to fill its valence shell. Similarly, atoms with one or two electrons more or fewer than a closed shell are highly reactive because they can either lose electrons to form positive ions or gain electrons to form negative ions, or share electrons through covalent bonds.