What are ions, and how do you predict their charge?

Atoms are neutral, ions are not

An atom has positively charged protons in its nucleus and negatively charged electrons around it. In a neutral atom, the numbers are equal, so the charges cancel out. A sodium atom has 11 protons and 11 electrons, so its overall charge is zero.

In a chemical reaction, the nucleus does not change, but electrons can move from one atom to another. If an atom ends up with a different number of electrons than protons, it has an overall charge and is called an ion. You can find the charge with one subtraction: charge = protons − electrons.

Cations and anions

When an atom loses electrons, it has more protons than electrons, so its charge is positive. A positive ion is called a cation. Sodium loses 1 electron: 11 protons − 10 electrons = +1, so the ion is Na⁺. Magnesium loses 2 electrons: 12 protons − 10 electrons = +2, written Mg²⁺.

When an atom gains electrons, it has more electrons than protons, so its charge is negative. A negative ion is called an anion. Chlorine gains 1 electron: 17 protons − 18 electrons = −1, so the ion is Cl⁻. Oxygen gains 2 electrons: 8 protons − 10 electrons = −2, written O²⁻.

To write a charge, put the number first and the sign second, as a superscript: Mg²⁺, not Mg⁺². If the charge is 1, leave out the number, as in Na⁺ and Cl⁻.

Why atoms gain or lose electrons

Atoms are most stable with a full outer shell, like the noble gases. Sodium’s electrons are arranged 2, 8, 1. Losing its single outer electron leaves 2, 8, the same arrangement as neon. Gaining 7 electrons instead would take far more energy, so sodium loses 1.

Chlorine is arranged 2, 8, 7. Gaining just 1 electron gives it 2, 8, 8, the same as argon. In general, metals have only a few outer electrons and tend to lose them, forming cations, while nonmetals have many outer electrons and tend to gain more, forming anions.

Predicting charges from the group

For main-group elements, the periodic table tells you the usual charge. Group 1 metals form +1 ions (Li⁺, Na⁺, K⁺), group 2 metals form +2 ions (Mg²⁺, Ca²⁺) and aluminum, in group 13, forms Al³⁺.

On the right side, count how many electrons each atom needs to reach 8. Group 15 elements have 5 outer electrons and need 3 more, so they form −3 ions (N³⁻). Group 16 elements need 2 more and form −2 ions (O²⁻, S²⁻), and group 17 elements need 1 more and form −1 ions (F⁻, Cl⁻, Br⁻). Noble gases already have a full shell, so their charge is 0. Carbon and silicon in group 14 usually share electrons instead of forming simple ions.

Many transition metals can form more than one ion. Iron forms Fe²⁺ and Fe³⁺, and copper forms Cu⁺ and Cu²⁺. Their names use a Roman numeral to show the charge: iron(II) means Fe²⁺ and iron(III) means Fe³⁺.

Naming ions and combining them

A cation keeps its element’s name: Na⁺ is the sodium ion and Ca²⁺ is the calcium ion. A single-atom anion changes its ending to -ide: chlorine becomes chloride (Cl⁻), oxygen becomes oxide (O²⁻), sulfur becomes sulfide (S²⁻) and nitrogen becomes nitride (N³⁻).

In a compound, the positive and negative charges must add up to zero. Na⁺ and Cl⁻ balance one to one, giving NaCl, table salt. Calcium chloride needs two Cl⁻ ions for every Ca²⁺, so its formula is CaCl₂. Aluminum oxide needs two Al³⁺ ions (+6 in total) and three O²⁻ ions (−6 in total), so it is Al₂O₃.