What are valence electrons and how do you find them?

What valence electrons are

The electrons in an atom are arranged in shells around the nucleus. Electrons in the inner shells, called core electrons, are held tightly and rarely take part in reactions. Electrons in the outermost shell are farther from the nucleus, less tightly held and the first to interact with other atoms. These are the valence electrons.

Sodium has 11 electrons arranged as 1s² 2s² 2p⁶ 3s¹. The first ten form a stable core with the same arrangement as neon, and the single 3s electron is sodium’s only valence electron. Almost all of sodium’s chemistry comes from that one electron.

How to find valence electrons from the group number

For main-group elements, the periodic table gives you the answer directly. Elements in groups 1 and 2 have 1 and 2 valence electrons. For groups 13 to 18, subtract 10 from the group number: group 13 has 3, group 14 has 4, group 15 has 5, group 16 has 6, group 17 has 7 and group 18 has 8.

So carbon (group 14) has 4 valence electrons, nitrogen (group 15) has 5, oxygen (group 16) has 6 and neon (group 18) has 8. Helium is the exception in group 18: it has only 2 valence electrons, which completely fill its first shell.

How to find them from the electron configuration

If you know the electron configuration, count the electrons in the highest-numbered shell. For main-group elements, these are the outer s and p electrons.

Oxygen is 1s² 2s² 2p⁴. Its highest shell is 2, which holds 2 + 4 = 6 electrons, so oxygen has 6 valence electrons. Bromine is [Ar] 3d¹⁰ 4s² 4p⁵. Its highest shell is 4, which holds 2 + 5 = 7 electrons. The full 3d subshell belongs to an inner shell and is not counted, so bromine has 7 valence electrons, like every other halogen.

Valence electrons and ions

Atoms tend to gain, lose or share electrons until they reach a stable arrangement, usually eight outer electrons (an octet) like a noble gas. Very small atoms such as hydrogen and lithium aim for helium’s two instead.

Elements with few valence electrons lose them to form positive ions, and elements with many gain electrons to form negative ions. Group 1 metals form +1 ions (Na⁺), group 2 metals form +2 ions (Mg²⁺) and aluminum in group 13 forms +3 ions (Al³⁺). On the other side, nitrogen and phosphorus in group 15 form −3 ions (N³⁻, P³⁻), oxygen and sulfur in group 16 form −2 ions (O²⁻, S²⁻) and the halogens in group 17 form −1 ions (Cl⁻). Noble gases already have a full outer shell, so their charge is 0. Carbon and silicon in group 14 usually share their 4 valence electrons in covalent bonds instead of forming simple ions.

Valence electrons and bonding

Valence electrons also explain covalent bonds. Carbon has 4 valence electrons and needs 4 more to complete an octet, so it usually forms 4 bonds, as in methane (CH₄). Nitrogen, with 5, forms 3 bonds in ammonia (NH₃), and oxygen, with 6, forms 2 bonds in water (H₂O).

Lewis dot structures show valence electrons as dots around an element’s symbol. Oxygen is drawn with 6 dots and chlorine with 7, which makes it easy to see how many electrons each atom still needs.

Transition metals: a special case

For transition metals the rule is less simple, because electrons in the d subshell just below the outer shell can also take part in bonding. On this site, the valence electron count shown for a transition metal is the number of electrons in its outer s subshell. Iron, [Ar] 3d⁶ 4s², is shown with 2 valence electrons.

Keep in mind that the d electrons bond too. Iron commonly forms both Fe²⁺ and Fe³⁺: it loses its two 4s electrons first, and then a 3d electron to become Fe³⁺. This is why many transition metals have more than one common charge.