In chemistry and physics, a valence electron is an electron associated with an atom that can form a chemical bond and participate in a chemical reactions. Valence electrons are outer shell electrons for main group elements. For the transition metals with partially-filed d shells, valence electrons are those electrons outside the noble gas core. The number of valence electrons indicates the maximum number of chemical bonds an atom can form.
Number of Valence Electrons
For main group elements, the number of valence electrons usually ranges between 1 and 8 because eight electrons forms a complete octet. Elements from groups have preferred numbers of valence electrons. For example, alkali metal atoms (e.g., lithium, sodium) have one valence electron. Alkaline earth atoms (e.g., magnesium, calcium) have two valence electrons. The noble gases have complete octets, so all eight of their electrons are valence electrons. The exception is helium, which has two valence electrons.
The transition metals make use of the d-subshell, which can accommodate 10 electrons. The f-subshell holds 14 electrons and the g-subshell contains up to 18 electrons. Metals in the middle of the periodic table become more stable by emptying a shell, half-filling it, or completely filling it. So, they can have more than 8 valence electrons.
How to Find the Number of Valence Electrons
The easiest way to find the number of valence electrons is to go by the element group in the valence periodic table. However, the most common method uses atom’s ground state electron configuration. For main group elements, you’re looking for the number of electrons in the highest principal quantum number or the highest shell number. For example, in 1s22s2 (helium), 2 is the highest quantum number. There are two 2s electrons, so a helium atom has two valence electrons. For transition metals, the number of valence electrons is the number of electrons in subshells past the atom’s noble gas core. For example, the electron configuration of scandium is [Ar]3d14s2, for a total of 3 valence electrons.
- Magnesium’s ground state electron configuration is 1s22s2p63s2, the valence electrons would be the 3s electrons because 3 is the highest principal quantum number. Magnesium has two valence electrons.
- Carbon’s ground state electron configuration is 1s22s22p2. The highest principal quantum number is 2. There are 2 electrons in the 2s subshell and 2 electrons in the 2 p subshell, giving carbon a total of four valence electrons.
- Bromine’s ground state electron configuration is 1s22s2p63s2p6d104s24p5. The valence electrons are be the 4s and 4p electrons. Bromine has seven valence electrons.
- The electron configuration of an iron atom is 1s22s22p63s23p64s23d6 or [Ar]4s23d6. Iron is a transition metal, so the number of valence electrons includes those in the 3d subshell, not just those in the 4s subshell. There are two electrons in the 4s subshell and 6 electrons in the 3d subshell, so iron has 8 valence electrons.
Valency vs Oxidation State
Valency is the number of electrons in an atom’s outermost electron shell. Oxidation state reflects the number of electrons that an atom actually can gain, lose, or share with another atom. The number of valence electrons indicates the maximum number of chemical bonds an atom can form, while oxidation state does not. Valency does not indicate electrical charge, while oxidation state does.
The number of valence electrons in an atom may have the same or different numerical value as its oxidation state. For example, a lithium atom has 1 valence electron and has an oxidation state of +1. In contrast, a neon atom has 8 valence electrons and an oxidation state of 0. A hydrogen atom has 1 valence electron. It has an oxidation state of +1 when it combines with most elements, but an oxidation state of -1 when it forms a compound with an alkali metal. The oxidation state of a pure element is always zero, but the number of valence electrons is not zero.
- IUPAC (1997). “Valence”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.V06588
- Miessler G.L.; Tarr, D.A. (1999). Inorganic Chemistry (2nd ed.) Prentice-Hall.
- Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry: Principles and Modern Applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. ISBN 978-0-13-014329-7.