Ions: cations, anions and charge
What is an ion? How cations and anions form, how to work out the charge on an ion from the periodic table, and where ions matter in peptide chemistry.
By Zachery Casey · Published · Reviewed · Editorial policy
An ion is an atom or a group of atoms that carries an electric charge. An atom becomes an ion when it loses or gains electrons. Ions are everywhere in a lab: in salt solutions, in buffers, in the powder of a synthetic peptide and inside a mass spectrometer. This guide explains what ions are, the difference between cations and anions, how to work out the charge on an ion, how ions behave when dissolved, and where they turn up in peptide chemistry.
Terms are defined in the glossary.
What is an ion?
A neutral atom has the same number of protons, which are positive, as electrons, which are negative. The charges cancel out.
If the numbers become unequal, the atom carries an overall charge. It is then an ion. The charge equals the number of protons minus the number of electrons (OpenStax Chemistry 2e):
- A sodium atom has 11 protons and 11 electrons. If it loses one electron, it has 11 protons and 10 electrons. It becomes the sodium ion, Na⁺, with a charge of 1+.
- An oxygen atom has 8 protons. If it gains two electrons, it has 10 electrons and becomes O²⁻, with a charge of 2−.
The nucleus doesn't change. Only the electrons do, so an ion is still the same element.
Cations and anions
| Cation | Anion | |
|---|---|---|
| Charge | Positive | Negative |
| How it forms | Loses electrons | Gains electrons |
| Typical elements | Metals, such as sodium, potassium, calcium | Non-metals, such as chlorine and oxygen |
| Moves towards | The negative electrode (cathode) | The positive electrode (anode) |
| Examples | Na⁺, K⁺, Ca²⁺, NH₄⁺ | Cl⁻, OH⁻, CH₃COO⁻, PO₄³⁻ |
In short, metals tend to form cations and non-metals tend to form anions. The next section shows how to predict the size of the charge.
How to work out the charge on an ion
For the main-group elements, the group number in the periodic table predicts the usual charge. Atoms lose or gain just enough electrons to end up with the same number of electrons as the nearest noble gas. Metals lose electrons and non-metals gain them (OpenStax Chemistry 2e):
| Group | Electrons lost or gained | Usual charge | Example |
|---|---|---|---|
| 1 | Loses 1 | 1+ | Na⁺, K⁺ |
| 2 | Loses 2 | 2+ | Mg²⁺, Ca²⁺ |
| 13 | Loses 3 | 3+ | Al³⁺ |
| 15 | Gains 3 | 3− | N³⁻ |
| 16 | Gains 2 | 2− | O²⁻, S²⁻ |
| 17 | Gains 1 | 1− | Cl⁻, F⁻ |
| Transition metals | Varies | Often more than one | Cu⁺ and Cu²⁺, Fe²⁺ and Fe³⁺ |
Transition metals are the exception. Their charges can't be read off the table, and many form more than one ion: copper forms both Cu⁺ and Cu²⁺, and iron forms both Fe²⁺ and Fe³⁺ (OpenStax Chemistry 2e). That is why their compounds carry a Roman numeral in the name: iron(II) chloride, FeCl₂, contains Fe²⁺, and iron(III) chloride, FeCl₃, contains Fe³⁺ (OpenStax Chemistry 2e).
For a compound, the charges must cancel out. Sodium chloride is Na⁺ with Cl⁻, so the formula is NaCl. Magnesium chloride is Mg²⁺ with two Cl⁻, so the formula is MgCl₂.
Cation vs anion: an easy way to remember
Two short memory aids cover it:
- Cations are "paws-itive". Think of a cat's paws: a cation has a positive charge. The "t" in "cation" also looks like a plus sign.
- An anion is "a negative ion". The first letters, A-N, start both "anion" and "a negative".
The electrodes follow the same pattern: cations move to the cathode, and anions move to the anode.
Monatomic and polyatomic ions
A monatomic ion is a single charged atom, such as Na⁺ or Cl⁻.
A polyatomic ion is a group of atoms joined by covalent bonds that carries an overall charge and acts as one unit (OpenStax Chemistry 2e). Many of the ions in a lab are polyatomic:
| Ion | Formula | Where you meet it |
|---|---|---|
| Ammonium | NH₄⁺ | Ammonium acetate and bicarbonate buffers |
| Hydroxide | OH⁻ | Bases such as sodium hydroxide |
| Acetate | CH₃COO⁻ | Acetic acid solutions and acetate salts |
| Trifluoroacetate | CF₃COO⁻ | The counter-ion on most synthetic peptides |
| Phosphate | PO₄³⁻, HPO₄²⁻, H₂PO₄⁻ | Phosphate buffers |
| Bicarbonate | HCO₃⁻ | Volatile LC-MS buffers |
Ions in solution
When an ionic compound such as sodium chloride dissolves in water, it splits into its separate ions. This is dissociation. Ionic compounds dissociate almost completely, so their solutions conduct electricity well. Substances like this are called strong electrolytes (OpenStax Chemistry 2e).
Water helps by surrounding each ion. The slightly negative oxygen atoms of water face the cations, and the slightly positive hydrogen atoms face the anions. For why water molecules are lopsided in this way, and how ions are held together in a solid by ionic bonds, see the chemical bonds guide.
Acids and bases also produce ions. An acid gives up hydrogen ions (H⁺) in water, and a base takes them up or releases hydroxide ions. See acids and bases.
Ions in peptide chemistry
Charged groups on a peptide
A peptide isn't a simple ion, but it carries charged groups. Its amine groups can pick up a proton and become positive, and its carboxylic acid groups can lose one and become negative. The overall charge depends on pH. The pH at which the charges balance is the isoelectric point. See isoelectric point and net charge.
Counter-ions and peptide salts
A peptide with positive charges can't exist alone as a solid. Each positive charge is paired with a negative ion, called a counter-ion. Most synthetic peptides are supplied with trifluoroacetate as the counter-ion, because trifluoroacetic acid is used to make and purify them. Some are exchanged to acetate or chloride. The counter-ion adds to the weight of the powder. See trifluoroacetic acid and counter-ions.
Separating by charge
Ion-exchange chromatography separates molecules by their charge. The column carries fixed charged groups that hold on to ions of the opposite charge. A rising salt concentration then releases them one by one. See how peptides are purified.
Ions in a mass spectrometer
A mass spectrometer can only measure ions. Electrospray ionisation turns peptides into ions by adding protons, often more than one. The same peptide then appears at several mass-to-charge (m/z) values, one for each charge state. See LC-MS, electrospray and MALDI.
Related guides
For atoms that differ in mass rather than charge, see what is an isotope?. For how ionic bonds compare with covalent ones, see chemical bonds and polarity. For reactions in which atoms and ions gain or lose electrons, see oxidation and reduction. For dissolving salts and other solids, see solvents, solutes and solutions.
More on peptide chemistry
- Energy in reactions: enthalpy, exothermic and endothermic reactions, and Gibbs free energyWhat enthalpy is, how exothermic and endothermic reactions differ, how Gibbs free energy decides if a reaction can happen, and Le Chatelier's principle.
- Rates of reaction: what speeds a reaction up or slows it downWhat a rate of reaction is, how to calculate it, what changes it, how zero-, first- and second-order reactions differ, and how to work out a half-life.
- Moles, Avogadro's number and percentage yieldPercentage yield and theoretical yield with worked examples, plus what a mole is, what Avogadro's number means and how to calculate moles from mass.