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.

An ion forms when the number of electrons differs from the number of protons.

Protons are in the nucleus and usually do not move during simple chemical processes. The electron count changes when an ion forms. The number of protons determines the element, while the difference between proton and electron counts determines the ion charge.

Each proton contributes $+1$ and each electron contributes $-1$ . The ion charge is therefore the proton count minus the electron count. Equal counts give a neutral atom. Fewer electrons give a positive ion, while more electrons give a negative ion.

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.

An ion is an atomic or molecular particle with a net electric charge.

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: $+1$

Electron count changes: Loses $1$ electron
Electrons: $e^-: 11 \rightarrow 10$

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

Ion Charge from the Difference between Proton and Electron Counts

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 $10$ .

Composition of an Ion

When an atom becomes an ion, its proton and neutron counts stay the same. Its electron count changes according to the ion's charge. The table shows what happens to each part.

Part to read	When an ion forms	Effect
Protons	Stay the same	The element does not change.
Neutrons	Stay the same for the same isotope	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 $11$ . A neutral sodium atom has $11$ electrons. After losing one electron, $\mathrm{Na}^{+}$ has $10$ .

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.