Sciences

Ion: An Atom That Has Lost or Gained Electrons

An ion is a charged atom: sodium loses one electron to give Na⁺, chlorine gains one to give Cl⁻. The group number tells you the charge before you count.

Ion

An ion is an atom or group of atoms that has lost or gained electrons and therefore carries a positive or negative electrical charge.

Aussi appelé: Cation, for a positive ion, Anion, for a negative ion
Où les élèves le rencontrent: Grade 9 chemistry, when ionic bonding is introduced, and again in Grade 10 in electrolysis and in the flame and precipitate tests for ions.

La réponse en bref
An ion is an atom, or a group of joined atoms, that carries an electrical charge because it has lost or gained electrons. Sodium loses one electron and becomes Na⁺; chlorine gains one and becomes Cl⁻. The protons never change, so the atom stays the same element throughout.

Un exemple

Na (11 protons, 11 electrons) − 1 electron → Na⁺ (11 protons, 10 electrons)

A sodium atom has the electron arrangement 2,8,1. Losing that single outer electron leaves 2,8, a full outer shell, and the arithmetic behind the charge is simply 11 positive protons against 10 negative electrons: one positive overall. Chlorine runs the other way. It is 2,8,7, gains one electron to reach 2,8,8, and finishes with 17 protons against 18 electrons, so Cl⁻.

The charge is a subtraction

The charge on an ion is the number of protons minus the number of electrons, and it is worth doing that subtraction rather than memorising which way round the signs go. Electrons are negative, so losing them leaves a surplus of positive charge and taking them in creates a deficit. That is why metals, which lose electrons, form positive ions, and non-metals form negative ones.

The number written in the charge tells you how many electrons moved. Mg²⁺ is a magnesium atom that lost two, and O²⁻ is an oxygen atom that gained two. Nothing has happened to the nucleus in either case: an ion has exactly the protons and neutrons the atom had.

Reading the charge off the periodic table

For the main groups, the group number predicts the charge, because the group number is the number of outer-shell electrons. Groups 1 to 3 lose them, groups 5 to 7 take enough to fill the shell, and group 0 does neither, which is precisely why the noble gases are unreactive.

The pattern behind the list is that every one of these ions ends up with the electron arrangement of the nearest noble gas. Na⁺, Mg²⁺, O²⁻ and F⁻ all have ten electrons, arranged 2,8, which is neon's arrangement. That is why the charges stop where they do.

Group 1 forms 1+ — Na⁺, K⁺
Group 2 forms 2+ — Mg²⁺, Ca²⁺
Group 3 forms 3+ — Al³⁺
Group 5 forms 3− — N³⁻
Group 6 forms 2− — O²⁻, S²⁻
Group 7 forms 1− — Cl⁻, Br⁻
Group 0 forms no ions — the outer shell is already full
Transition metals vary — iron gives both Fe²⁺ and Fe³⁺, which is why their compounds are named iron(II) chloride and iron(III) chloride

Ions made of several atoms

Not every ion is a single atom. Sulfate is SO₄²⁻, nitrate is NO₃⁻, carbonate is CO₃²⁻ and hydroxide is OH⁻; ammonium, NH₄⁺, is the one common positive example. In each of these the atoms are bonded to one another and the charge belongs to the group as a whole rather than to any one atom in it.

This is where brackets appear in formulae. Calcium hydroxide is Ca(OH)₂ because two whole hydroxide ions are needed to balance one Ca²⁺, and the bracket keeps the pair together. Writing CaOH₂ says something different and wrong.

Questions fréquentes

What is the difference between an atom and an ion?

An atom has equal numbers of protons and electrons, so it has no overall charge. An ion has lost or gained electrons, so it has one. Nothing else differs — the nucleus is untouched, and a sodium ion is still sodium, which is why the symbol keeps the same letters and only adds a charge.

Is an ion the same as an isotope?

No, and they are easy to confuse because both are variations on an ordinary atom. An isotope has a different number of neutrons and the same electrons. An ion has a different number of electrons and the same neutrons. One changes the mass, the other changes the charge.

Why do metals form positive ions?

Because metal atoms have few electrons in their outer shell — one, two or three — and losing that handful is a shorter route to a full outer shell than gaining five or six would be. Losing negative electrons leaves the positive protons in the majority, so the ion carries a positive charge.

How do I know how many electrons an ion has?

Start from the atomic number, which gives the electrons in the neutral atom, then subtract the charge. Al³⁺ has atomic number 13, so 13 − 3 = 10 electrons. For a negative ion the charge is negative and subtracting it adds: O²⁻ is 8 − (−2) = 10 electrons.

Dernière mise à jour 21 août 2026