Ions

When an Atom Is No Longer Neutral

A neutral atom has the same number of protons and electrons. An ion forms when an atom or group of atoms has an overall charge. Here, we focus first on ions made from one atom.

The Atomic Structure and Symbolism section from OpenStax explains that ions form when the number of electrons is not equal to the number of protons. The source can be opened through openstax.org.

Protons are in the nucleus and usually do not move during simple chemical processes. Electrons are the particles that change. So element identity stays locked by the number of protons, while ion charge is determined by the difference between protons and electrons.

A simple way to read charge is by its sign. Protons add positive count, while electrons add negative count. If the counts match, the result is $0$ or neutral. If electrons decrease, positive charge remains. If electrons increase, negative charge remains.

Cations and Anions

A positively charged ion is called a cation. A cation forms when an atom loses electrons.

A negatively charged ion is called an anion. An anion forms when an atom gains electrons.

The IUPAC Gold Book defines an ion as an atomic or molecular particle that has a net electric charge. The term reference can be opened through goldbook.iupac.org.

Ion Formation Simulator

Follow the left-to-right path: a neutral atom loses or gains electrons, then becomes a cation or anion. Protons and neutrons stay the same.

Before

Neutral atom

{}^{23}_{11}\mathrm{Na}

Electrons: $11$
Charge: $0$

After

Cation

{}^{23}_{11}\mathrm{Na}^{+}

Electrons: $10$
Charge: $+1$

Electrons change: Loses $1$ electron
Electrons: $e^-: 11 \rightarrow 10$

Protons: $11$
Neutrons: $23 - 11 = 12$
Charge: $q = p^+ - e^- = 11 - 10 = +1$

Charge Comes from the Difference

The safest formula for reading ion charge is:

q = p^+ - e^-

The symbol $q$ is the ion charge, $p^+$ is the number of protons, and $e^-$ is the number of electrons.

To find the electron count of an ion, rearrange the same formula:

e^- = p^+ - q

Sodium example:

\begin{aligned} {}^{23}_{11}\mathrm{Na}^{+} &: p^+ = 11,\ q = +1 \\ e^- &= 11 - (+1) \\ &= 10 \end{aligned}

Fluoride example:

\begin{aligned} {}^{19}_{9}\mathrm{F}^{-} &: p^+ = 9,\ q = -1 \\ e^- &= 9 - (-1) \\ &= 10 \end{aligned}

The negative sign in $\mathrm{F}^{-}$ means there are more electrons than protons. That is why $9 - (-1)$ becomes $10$ .

What Changes and What Stays

When an atom becomes an ion, do not change every number at once. Read each part separately.

Part to read	When an ion forms	Effect
Protons	Stay the same	The element does not change.
Neutrons	Stay the same for the same ion	The mass number does not change.
Electrons	Change	The ion charge changes.

So $\mathrm{Na}$ and $\mathrm{Na}^{+}$ are both sodium because the proton count stays $11$ . The difference is that $\mathrm{Na}^{+}$ has $10$ electrons, not $11$ electrons.

Worked Table for Ion Counts

Use the table below as a completed example. Read each row from left to right: atomic number $Z$ gives the proton count, $A - Z$ gives the neutron count, and $q = p^+ - e^-$ connects charge with the electron count.

Symbol	Atomic number	Mass number	Charge	Protons	Electrons	Neutrons
${}^{7}_{3}\mathrm{Li}$	$3$	$7$	$0$	$3$	$3$	$4$
${}^{7}_{3}\mathrm{Li}^{+}$	$3$	$7$	$+1$	$3$	$2$	$4$
${}^{16}_{8}\mathrm{O}$	$8$	$16$	$0$	$8$	$8$	$8$
${}^{16}_{8}\mathrm{O}^{2-}$	$8$	$16$	$-2$	$8$	$10$	$8$
${}^{27}_{13}\mathrm{Al}$	$13$	$27$	$0$	$13$	$13$	$14$
${}^{27}_{13}\mathrm{Al}^{3+}$	$13$	$27$	$+3$	$13$	$10$	$14$

Checking the Lithium Row

Check one row, such as ${}^{7}_{3}\mathrm{Li}^{+}$ . Its atomic number is $3$ , so it has $3$ protons. Its charge is $+1$ , so the electron count is:

e^- = 3 - (+1) = 2

Its neutron count comes from the mass number:

n^0 = 7 - 3 = 4

If you feel unsure, start from the charge calculation. Cations always have fewer electrons than protons, while anions have more electrons than protons.