What is an ion?

What is an ion?

Every atom contains positive protons in its nucleus and the same number of negative electrons around it. The charges cancel, so an atom is neutral. Electrons are the only particles that move during chemical reactions, though. If an atom loses or gains some, the protons and electrons no longer balance, and the atom has an overall charge. It is now an ion.

The names cation and anion come from electrolysis: cations move towards the negative electrode, the cathode, and anions towards the positive one, the anode. A memory trick: a cation is "pawsitive", and an anion is a negative ion.

Why do atoms form ions?

Atoms are most stable with a full outer shell of electrons, like the noble gases in group 0. Metals have only one, two or three outer electrons, so it is easiest for them to lose those and be left with the full shell underneath. Non-metals have nearly full outer shells, so it is easiest for them to gain a few. Either way, the ion ends up with the same electron arrangement as a noble gas.

Magnesium, for example, has the arrangement 2,8,2. It loses its 2 outer electrons to become Mg²⁺, with the arrangement 2,8, the same as neon. Oxygen, 2,6, gains 2 electrons to become O²⁻, also 2,8. You can add and remove electrons yourself in our Atom Builder and watch the charge change.

How to work out the charge of an ion

Each proton counts +1 and each electron −1. An ion with 19 protons and 18 electrons has a charge of 19 − 18 = 1+: it is a potassium ion, K⁺. An ion with 34 protons and 36 electrons has a charge of 34 − 36 = 2−: a selenide ion, Se²⁻.

Reading ion charges from the periodic table

For the main groups, the group number tells you the charge, as the AQA GCSE specification expects you to work out:

Group 4 elements such as carbon rarely form simple ions, and group 0 elements don't form ions at all, because their shells are already full.

Protons, neutrons and electrons in an ion

Use the periodic table for protons and neutrons, then adjust the electrons for the charge. Protons = atomic number. Neutrons = mass number − atomic number. Electrons = protons − charge, so a positive ion has fewer electrons than protons and a negative ion has more.

Notice that Al³⁺ and F⁻ both end up with 10 electrons, the same as a neon atom, even though they are completely different elements. The number of neutrons never changes when an ion forms.

Try one: this question gives you an atom and asks you to add or remove electrons to make its ion.

How to write and name ions

• Write the charge as a superscript after the symbol, number first and then the sign: Mg²⁺, O²⁻. A charge of 1 is written as just + or −, as in Na⁺ and Cl⁻.
• Positive metal ions keep the element's name: sodium ion, calcium ion.
• Negative ions of single elements end in "ide": chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide, nitrogen becomes nitride.
• Transition metals can form more than one ion, so the charge goes in Roman numerals: iron(II) is Fe²⁺ and iron(III) is Fe³⁺.

Polyatomic ions

Some ions are groups of atoms bonded together, with an overall charge shared by the whole group. They stay together through reactions, so it is worth learning the common ones:

Where ions matter

• Ionic compounds such as table salt are made of positive and negative ions held together by their opposite charges. See our guide to ionic bonding.
• Electrolysis works because dissolved or molten ions can move and carry charge to the electrodes.
• Acids and alkalis: acids release hydrogen ions, H⁺, in water, and alkalis release hydroxide ions, OH⁻.
• Your body relies on ions such as sodium, potassium and calcium ions for nerve signals and muscle contractions.
• Ionising radiation gets its name from knocking electrons out of atoms, turning them into ions.

Ion vs atom vs isotope

Common mistakes

• Changing the number of protons. Only electrons are lost or gained. Changing protons would make a different element.
• Getting the sign backwards. Losing negative electrons makes an ion positive.
• Writing the charge as +2 instead of 2+. In ion formulae, the number comes first: Mg²⁺.
• Calling Cl⁻ a chlorine ion. It is a chloride ion.
• Changing the neutrons. Forming an ion doesn't affect the nucleus at all.
• Mixing up ions and isotopes. Ions differ in electrons; isotopes differ in neutrons.

Read next

Chemistry · Atoms and the periodic tableElectron configuration calculator: any element or ionFree electron configuration calculator: full and noble gas configurations of any element or ion, with orbital box diagrams and the Cr and Cu exceptions.