Ions: Cations, Anions, and Net Charges

An ion is an atom or group of atoms that has gained or lost electrons and so carries a net electric charge: a cation is a positive ion formed by losing electrons, and an anion is a negative ion formed by gaining electrons. In brief: a sodium atom (11 protons, 11 electrons) loses one electron to become Na⁺ (11 protons, 10 electrons), and a chlorine atom gains one electron to become Cl⁻ (17 protons, 18 electrons); the net charge is protons minus electrons.

Ions are central to chemistry. They are the building blocks of salts, the carriers of current in solutions and the active species in nerve signals and batteries. This article explains how ions form, how to calculate the net charge and the number of particles in an ion, which charges are typical for each group of the periodic table, includes tables of common ions, works through examples and gives the history of the concept.

How do ions form?

Atoms become ions by transferring electrons. The number of protons in the nucleus does not change, so the atom’s identity stays the same, but the balance between positive and negative charge is disturbed.

• Cations form when an atom loses one or more electrons. Metals usually form cations, because they have few outer electrons that are easy to remove. A magnesium atom loses two electrons: Mg → Mg²⁺ + 2e⁻.
• Anions form when an atom gains one or more electrons. Nonmetals usually form anions. An oxygen atom gains two electrons: O + 2e⁻ → O²⁻.

Atoms tend to form ions with a stable electron arrangement like that of the nearest noble gas, often eight outer electrons. The cation is smaller than its atom, since it has fewer electrons and often loses a whole shell, and an anion is larger, since the added electrons repel one another.

How do you calculate the net charge?

Net charge = number of protons − number of electrons. Each proton has a charge of +1 and each electron has −1. If protons exceed electrons, the ion is positive. If electrons exceed protons, it is negative. The charge is written as a superscript with the number first and then the sign: Mg²⁺, Al³⁺, O²⁻, and a charge of one is written without the number: Na⁺, Cl⁻.

Which charges do elements usually form?

Particles in common ions

Notice that Na⁺, Mg²⁺, Al³⁺ and O²⁻ all have 10 electrons, the same as neon. Species with the same number of electrons are called isoelectronic.

What about transition metals and polyatomic ions?

Many transition metals form more than one ion. Iron forms Fe²⁺ (iron(II)) and Fe³⁺ (iron(III)), and copper forms Cu⁺ and Cu²⁺. The charge is shown by a Roman numeral in the name. Polyatomic ions are groups of covalently bonded atoms that carry a net charge, such as ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, sulfate SO₄²⁻ and carbonate CO₃²⁻. They behave as single units in reactions.

Worked examples

1. Net charge from particle counts. A particle has 20 protons and 18 electrons. Charge = 20 − 18 = +2, so it is Ca²⁺.
2. Electrons in an ion. ⁵⁶₂₆Fe³⁺ has 26 protons, 56 − 26 = 30 neutrons and 26 − 3 = 23 electrons.
3. Anion. A phosphide ion P³⁻ comes from phosphorus (Z = 15). The ion has 15 + 3 = 18 electrons, the same as argon.
4. Ionic compound charge balance. In magnesium chloride, Mg²⁺ combines with Cl⁻. Two chloride ions balance one magnesium ion: +2 + 2(−1) = 0, so the formula is MgCl₂.
5. Mass of an ion. The mass lost when a sodium atom becomes Na⁺ is the mass of one electron, 0.000549 u, which is 0.0024% of the mass of 22.99 u, so the mass of an ion is practically the mass of the atom.
6. Charge of a sample. One mole of Na⁺ ions carries a charge of 6.022 × 10²³ × 1.602 × 10⁻¹⁹ C = 96,485 C, the Faraday constant.

Why are ions important?

• Ionic compounds: table salt, limestone and many minerals are lattices of ions.
• Conduction: molten ionic compounds and aqueous solutions of ions conduct electricity, which is the basis of electrolysis and batteries.
• Biology: sodium, potassium, calcium and chloride ions carry nerve signals, regulate muscle contraction and balance fluids.
• Water quality: hardness is caused by calcium and magnesium ions.
• Technology: lithium ions move between electrodes in rechargeable batteries.

Where do the terms come from?

Michael Faraday introduced the words ion, cation, anion, electrode and electrolyte in the 1830s, with the help of William Whewell, from Greek words meaning “going”, “going down” and “going up”. Faraday did not know what ions were. Svante Arrhenius proposed in 1884 that salts dissociate into charged particles in water, and the idea was at first controversial. It earned him the Nobel Prize in Chemistry in 1903, after the discovery of the electron in 1897 helped to explain how atoms could become charged.

Which mistakes are common?

Mistake 1: Saying that a cation has gained protons

Ions form by changes in electrons only. The number of protons is fixed.

Mistake 2: Mixing up cations and anions

Remember that cations are positive (“cat-ions are paws-itive”), and anions are negative.

Mistake 3: Writing the charge in the wrong order

The number goes before the sign: Mg²⁺, not Mg⁺², in standard notation.

Mistake 4: Thinking that ions are always dissolved

Solid ionic compounds contain ions in a fixed lattice. They are free to move only when molten or dissolved.