Valence Electrons

Valence Electrons in Chemical Reactions

Start with calcium. In the simple shell model, a neutral calcium atom is written with the electron configuration $2, 8, 8, 2$ . That means calcium's electrons are spread across the $\mathrm{K}$ , $\mathrm{L}$ , $\mathrm{M}$ , and $\mathrm{N}$ shells.

The outermost occupied shell is the $\mathrm{N}$ shell. It contains $2$ electrons. So, a calcium atom has $2$ valence electrons.

Valence electrons are the electrons in the outermost occupied shell of an atom.

When atoms form bonds or ions, they most often share, lose, or gain these outer electrons. The number and arrangement of valence electrons therefore affect how an element reacts.

Valence Electron Model

Choose a neutral atom. Read the shell configuration from left to right, then look at the last shell that still contains electrons. The number in that last shell is the number of valence electrons.

Valence Electron Reader

The outer shell uses a different color so the valence-electron count is visible directly from the model.

Its configuration is $2, 8, 8, 2$ . The $\mathrm{N}$ shell contains $2$ electrons.

Valence electrons: $2$
Outermost shell: $\mathrm{N}$
Shell configuration: $2, 8, 8, 2$
Tendency: Calcium tends to lose $2$ valence electrons when forming $\mathrm{Ca^{2+}}$ .

Identifying Valence Electrons from the Outermost Shell

The following steps determine the number of electrons in the outermost occupied shell.

Write the shell configuration for the neutral atom.
Find the last shell that still contains electrons.
The number of electrons in that last shell is the number of valence electrons.

Examples:

Atom	Shell configuration	Outermost shell	Valence electrons
$\mathrm{Mg}$	$2, 8, 2$	$\mathrm{M}$	$2$
$\mathrm{Cl}$	$2, 8, 7$	$\mathrm{M}$	$7$
$\mathrm{Ar}$	$2, 8, 8$	$\mathrm{M}$	$8$
$\mathrm{Ca}$	$2, 8, 8, 2$	$\mathrm{N}$	$2$

Do not add all electrons when looking for valence electrons. Calcium has $20$ total electrons, but only $2$ valence electrons because only the outermost occupied shell is counted.

Patterns in the Same Group

Main-group elements in the same group often have the same number of valence electrons. That is why their chemical properties can be similar.

The table orders hydrogen through francium by shell configuration:

Element	Atomic number	$\mathrm{K}$	$\mathrm{L}$	$\mathrm{M}$	$\mathrm{N}$	$\mathrm{O}$	$\mathrm{P}$	$\mathrm{Q}$	Valence electrons
$\mathrm{H}$	$1$	$1$	-	-	-	-	-	-	$1$
$\mathrm{Li}$	$3$	$2$	$1$	-	-	-	-	-	$1$
$\mathrm{Na}$	$11$	$2$	$8$	$1$	-	-	-	-	$1$
$\mathrm{K}$	$19$	$2$	$8$	$8$	$1$	-	-	-	$1$
$\mathrm{Rb}$	$37$	$2$	$8$	$18$	$8$	$1$	-	-	$1$
$\mathrm{Cs}$	$55$	$2$	$8$	$18$	$18$	$8$	$1$	-	$1$
$\mathrm{Fr}$	$87$	$2$	$8$	$18$	$32$	$18$	$8$	$1$	$1$

The number of occupied shells increases down the group, while the outermost shell still contains $1$ electron. This is why lithium, sodium, potassium, rubidium, cesium, and francium are placed in the same group.

Hydrogen is an exception. It has $1$ valence electron, but it does not behave like an alkali metal. Valence-electron count is one clue to chemical behavior, and the rest of the atomic structure also matters.

Connection to Chemical Properties

Chemical reactions often involve valence electrons, the electrons in the outer shell. Atoms can lose, gain, or share these electrons.

In short:

In many main-group metals, atoms with $1$ or $2$ valence electrons tend to lose electrons when forming ions.
Elements with $7$ valence electrons often gain or share $1$ electron.
Noble gases such as helium and argon have full outer shells, so they are very unreactive.

For main-group elements, the quick pattern is:

Group $1$ has $1$ valence electron.
Group $2$ has $2$ valence electrons.
Groups $13$ to $18$ have $3$ to $8$ valence electrons.

Helium is an important exception: helium is in group $18$ , but it has only $2$ valence electrons because its first shell is full with $2$ electrons.

For transition elements, electrons in the outermost $s$ subshell and nearby $d$ subshells can participate in bonding. The simple group-number shortcut therefore does not always give one valence-electron count. The examples below use main-group elements and the simple shell model.

Checking Valence Electrons from the Outermost Shell

Use this list as a check. Read the shell configuration, then take the rightmost shell count as the valence-electron count. The same count decides which group the element belongs to, so the two pieces of information should always agree.

First Twenty Elements

The table lists the shell configuration and the valence-electron count for each of the first twenty elements, so you can look up a single element and compare it with its neighbours in the same period or group.

Atomic number	Element	Shell configuration	Valence electrons
$1$	$\mathrm{H}$	$1$	$1$
$2$	$\mathrm{He}$	$2$	$2$
$3$	$\mathrm{Li}$	$2, 1$	$1$
$4$	$\mathrm{Be}$	$2, 2$	$2$
$5$	$\mathrm{B}$	$2, 3$	$3$
$6$	$\mathrm{C}$	$2, 4$	$4$
$7$	$\mathrm{N}$	$2, 5$	$5$
$8$	$\mathrm{O}$	$2, 6$	$6$
$9$	$\mathrm{F}$	$2, 7$	$7$
$10$	$\mathrm{Ne}$	$2, 8$	$8$
$11$	$\mathrm{Na}$	$2, 8, 1$	$1$
$12$	$\mathrm{Mg}$	$2, 8, 2$	$2$
$13$	$\mathrm{Al}$	$2, 8, 3$	$3$
$14$	$\mathrm{Si}$	$2, 8, 4$	$4$
$15$	$\mathrm{P}$	$2, 8, 5$	$5$
$16$	$\mathrm{S}$	$2, 8, 6$	$6$
$17$	$\mathrm{Cl}$	$2, 8, 7$	$7$
$18$	$\mathrm{Ar}$	$2, 8, 8$	$8$
$19$	$\mathrm{K}$	$2, 8, 8, 1$	$1$
$20$	$\mathrm{Ca}$	$2, 8, 8, 2$	$2$

Valence Electron Patterns within a Group

Elements $\mathrm{B}$ , $\mathrm{Al}$ , $\mathrm{Ga}$ , $\mathrm{In}$ , and $\mathrm{Tl}$ are in group $13$ . The quick main-group pattern is $3$ valence electrons.

$\mathrm{Na}$ and $\mathrm{K}$ sit in group $1$ , so the same pattern gives each $1$ valence electron that is easily lost, and both tend to form $+1$ ions. Argon is much less reactive than chlorine because argon's outer shell is full. Chlorine often gains one electron when forming an ion or shares electrons when forming covalent bonds.