Editing Valence electron (section)

==Chemical reactions==
{{main|Valence (chemistry)}}
The number of valence electrons in an atom governs its [[chemical bond|bonding]] behavior. Therefore, elements whose atoms have the same number of valence electrons are often grouped together in the [[periodic table]] of the elements, especially if they also have the same types of valence orbitals.<ref name=jensenlaw>{{cite web|url=http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/081.%20Periodic%20Table.pdf|last1=Jensen|first1=William B.|authorlink=William B. Jensen|title=The Periodic Law and Table|date=2000|archive-url=https://web.archive.org/web/20201110113324/http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/081.%20Periodic%20Table.pdf |access-date=10 December 2022|archive-date=2020-11-10 }}</ref>

The most [[reactivity (chemistry)|reactive]] kind of [[metallic element]] is an [[alkali metal]] of group 1 (e.g., [[sodium]] or [[potassium]]); this is because such an atom has only a single valence electron. During the formation of an [[ionic bond]], which provides the necessary [[ionization energy]], this one valence electron is easily lost to form a positive [[ion]] (cation) with a closed shell (e.g., Na<sup>+</sup> or K<sup>+</sup>). An [[alkaline earth metal]] of group 2 (e.g., [[magnesium]]) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion with a closed shell (e.g., Mg<sup>2+</sup>).{{citation needed|date=April 2023}}

Within each group (each periodic table column) of metals, reactivity increases with each lower row of the table (from a light element to a heavier element), because a heavier element has more electron shells than a lighter element; a heavier element's valence electrons exist at higher [[principal quantum number]]s (they are farther away from the nucleus of the atom, and are thus at higher potential energies, which means they are less tightly bound).{{citation needed|date=April 2023}}

A [[Nonmetal (chemistry)|nonmetal]] atom tends to attract additional valence electrons to attain a full valence shell; this can be achieved in one of two ways: An atom can either share electrons with a neighboring atom (a [[covalent bond]]), or it can remove electrons from another atom (an [[ionic bond]]). The most reactive kind of nonmetal element is a [[halogen]] (e.g., [[fluorine]] (F) or [[chlorine]] (Cl)). Such an atom has the following electron configuration: s<sup>2</sup>p<sup>5</sup>; this requires only one additional valence electron to form a closed shell. To form an ionic bond, a halogen atom can remove an electron from another atom in order to form an anion (e.g., F<sup>−</sup>, Cl<sup>−</sup>, etc.). To form a covalent bond, one electron from the halogen and one electron from another atom form a shared pair (e.g., in the molecule H–F, the line represents a shared pair of valence electrons, one from H and one from F).{{citation needed|date=April 2023}}

Within each group of nonmetals, reactivity decreases with each lower row of the table (from a light element to a heavy element) in the periodic table, because the valence electrons are at progressively higher energies and thus progressively less tightly bound. In fact, oxygen (the lightest element in group 16) is the most reactive nonmetal after fluorine, even though it is not a halogen, because the valence shells of the heavier halogens are at higher principal quantum numbers.

In these simple cases where the octet rule is obeyed, the [[valence (chemistry)|valence]] of an atom equals the number of electrons gained, lost, or shared in order to form the stable octet. However, there are also many molecules that are [[Octet rule#Exceptions|exceptions]], and for which the valence is less clearly defined.