Valence Electron

A valence electron is an electron in the part of an atom’s electronic structure that is principally involved in chemical bonding and chemical reactions. For main-group elements, valence electrons occupy the outermost occupied electron shell. In transition elements, electrons in an incompletely filled inner subshell may also have valence roles. Valence electrons are distinguished from core electrons, which generally remain comparatively unchanged during ordinary chemical processes. (openstax.org)

Electronic structure and counting

An electron configuration specifies the occupation of atomic orbitals. For a main-group atom, the valence shell is the occupied shell with the largest principal quantum number, $n$. Its electrons occupy $s$ and, except in the first period, $p$ subshells. Sodium, for example, has the configuration $[\mathrm{Ne}]3s^1$: the bracketed neon configuration represents ten core electrons, while the $3s$ electron is its single valence electron. (openstax.org)

For neutral main-group atoms, the periodic table provides a convenient counting rule: groups 1 and 2 have one and two valence electrons, respectively; groups 13–18 have three through eight. Helium is the exception in group 18, with a filled $1s^2$ shell containing two electrons. (openstax.org)

The recurrence of similar valence configurations helps explain the similar chemistry of elements within a group. (openstax.org)

Role in bonding and ion formation

In a covalent bond, atoms share valence-electron density. A single bond is represented by one shared electron pair, a double bond by two, and a triple bond by three. Valence electrons may also remain as nonbonding pairs, commonly called lone pairs, rather than being assigned to bonds. (openstax.org)

Formation of an ion changes the electron count. Main-group metals commonly lose valence electrons to form positive ions, while many nonmetals gain electrons to form negative ions. Sodium loses its $3s$ electron to form $\mathrm{Na^+}$; oxygen gains two electrons to form $\mathrm{O^{2-}}$, filling its $2p$ subshell. An ionic bond is the electrostatic attraction between oppositely charged ions, not simply the act of electron transfer. (openstax.org)

Loss and gain of electrons are central to oxidation–reduction reactions. Electron transfer also has an energetic cost or benefit: removing an electron requires ionization energy. Consequently, obtaining a filled shell is not by itself a complete explanation of whether a reaction occurs; the energetics of the whole process must be considered. (openstax.org)

Lewis structures and the octet rule

A Lewis structure represents valence electrons as bonding pairs, lone pairs, and sometimes unpaired electrons. For a main-group molecule or polyatomic ion, the total electron count is obtained by adding the neutral atoms’ valence-electron counts, adding one electron for each unit of negative charge, and subtracting one for each unit of positive charge. (openstax.org)

For example, water, $\mathrm{H_2O}$, has eight valence electrons in total: six from oxygen and one from each hydrogen. Its usual Lewis structure assigns four electrons to two O–H bonds and four to two lone pairs on oxygen. (openstax.org)

The octet rule describes the tendency of many main-group atoms to have eight electrons in their valence environment after bonding; hydrogen instead commonly has two. This is a useful rule rather than a universal law. Exceptions include odd-electron species, electron-deficient compounds, and structures conventionally drawn with more than eight electrons around a central atom. (openstax.org)

Transition and inner-transition elements

For transition elements, “valence electron” cannot always mean only “electron in the shell with the largest $n$.” Both outer $ns$ electrons and electrons in the $(n-1)d$ subshell may participate in chemical changes. Iron illustrates the distinction: neutral iron has the configuration $[\mathrm{Ar}]3d^6 4s^2$, whereas $\mathrm{Fe^{2+}}$ has $[\mathrm{Ar}]3d^6$, following removal of the two $4s$ electrons. (openstax.org)

Inner-transition elements may likewise lose outer $s$ electrons and electrons from $d$ or $f$ subshells. Thus, a stated valence-electron count for these elements needs a clear counting convention; the outermost-shell count alone does not describe all chemically relevant electrons. (openstax.org)

Distinction from valence

The number of valence electrons is not identical to an atom’s valence, or combining capacity. Oxygen has six valence electrons but commonly forms two covalent bonds. In the traditional definition, valence concerns the number of univalent atoms with which an atom or group can combine, whereas a valence-electron count describes electronic occupation. (openstax.org)

Valence electrons in solids

In a solid, interactions among many atoms produce electronic energy bands rather than merely isolated atomic energy levels. Valence electrons contribute to these extended states, so an atom-by-atom picture alone is insufficient to describe electrical conduction. (openstax.org)

In an intrinsic semiconductor, the valence band is filled at absolute zero and is separated from the empty conduction band by a band gap. Excitation across this gap creates a conduction-band electron and an electron hole in the valence band; both can carry current. Metals instead have partially filled bands or overlapping bands that provide accessible states for conduction. These distinctions show why conductivity depends on band occupation and structure, not simply on the number of valence electrons per atom. (openstax.org)

References

1. 4 Electronic Structure of Atoms (Electron Configurations) - Chemistry 2e | OpenStaxopenstax.org
2. 3 Lewis Symbols and Structures - Chemistry 2e | OpenStaxopenstax.org
3. 1 Ionic Bonding - Chemistry 2e | OpenStaxopenstax.org
4. 1 Occurrence, Preparation, and Properties of Transition Metals and Their Compounds - Chemistry | OpenStaxopenstax.org
5. Ch. 7 Summary - Chemistry | OpenStaxopenstax.org
6. 5 Band Theory of Solids - University Physics Volume 3 | OpenStaxopenstax.org