Valence Electrons

The Outer Electrons that React Most Often

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 electrons in the outermost occupied shell of an atom. OpenStax explains that outer-shell electrons are called valence electrons and play the most important role in chemical reactions. You can read the reference in openstax.org.

Because they are on the outside, valence electrons are the electrons most often involved when an atom bonds, loses electrons, or gains electrons. They do not simply fly away on their own. The point is that the outside of the atom strongly shapes how the atom 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 $\mathrm{Ca^{2+}}$ .

A Fast Way to Identify Valence Electrons

Use the same sequence every time so the idea stays clear.

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. OpenStax explains that elements in one group have the same number of valence electrons, and this shared number explains their similar chemical properties. You can read the reference in openstax.org.

Notice the pattern below.

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 table shows the key idea: even though the number of occupied shells increases down the group, the outermost shell still contains $1$ electron. This is why lithium, sodium, potassium, rubidium, cesium, and francium are placed in the same group.

Small caution: hydrogen also has $1$ valence electron, but it does not behave exactly like the alkali metals. The number of valence electrons is a strong clue, not the only explanation of an element's properties.

Connection to Chemical Properties

Valence electrons matter because chemical reactions often involve the outer electrons. Atoms can lose, gain, or share valence 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, counting valence electrons can be more complicated because $d$ subshells can matter. This discussion focuses on main-group elements and the simple shell model.

Outermost-Shell Self-Check

Use this list as a check. Read the shell configuration, then take the rightmost shell count as the valence-electron count.

First Twenty Elements

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

Patterns to Notice

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.

That is why $\mathrm{Na}$ and $\mathrm{K}$ both tend to form $+1$ ions: each has $1$ valence electron that is easily lost. Argon is much less reactive than chlorine because argon's outer shell is already full, while chlorine still needs $1$ more electron to reach a full-shell pattern.