Ions

An ion is an atom (or group of atoms) that has gained or lost one or more electrons, so it no longer has a balanced charge. A normal atom has the same number of negative electrons as positive protons, so its overall charge is zero. An ion is unbalanced: either more electrons than protons (overall negative) or fewer electrons than protons (overall positive). Ions are everywhere: in the salt on your chips, the water in your cells, the air after a thunderstorm and the wires of your phone battery.

  • What it isA charged atomOr group of atoms
  • CationPositively chargedLost one or more electrons
  • AnionNegatively chargedGained one or more electrons
  • Common cationsNa+, K+, Ca2+, H+Carry positive charge
  • Common anionsCl-, OH-, NO3-, SO4 2-Carry negative charge
  • Found inSalt, batteries, blood, oceansAlmost everywhere

How ions form

Ions form when atoms swap electrons. Why? Because atoms are happiest (most stable) when their outer electron shell is full. A full outer shell usually holds 8 electrons (2 in the case of helium-like atoms). Atoms with too few outer electrons get rid of them. Atoms with too many outer electrons grab a few more.

Take the classic example, table salt (sodium chloride):

  • Sodium has 11 electrons (2, 8, 1). It has only 1 in its outer shell. It would have to lose 1 to reach a full outer shell of 8.
  • Chlorine has 17 electrons (2, 8, 7). It has 7 in its outer shell. It would have to gain 1 to reach a full outer shell of 8.
  • So sodium hands its single outer electron to chlorine. Sodium becomes a Na+ ion (lost a negative electron, so positive overall). Chlorine becomes a Cl- ion (gained a negative electron, so negative overall).

The Na+ and Cl- ions strongly attract each other and stick together to form crystals of salt.

Cations and anions

There are two families of ions:

  • Cations (pronounced "cat-eye-ons"): positive ions. They have lost one or more electrons. Examples: Na+ (sodium), K+ (potassium), Ca2+ (calcium), Fe3+ (iron III).
  • Anions (pronounced "an-eye-ons"): negative ions. They have gained one or more electrons. Examples: Cl- (chloride), O2- (oxide), OH- (hydroxide), SO4 2- (sulfate).

One trick to remember: a CAT-ion is "paws-itive" (a tiny joke).

Fact Your body needs a careful balance of ions to work. Nerve signals travel along your nerves as waves of sodium (Na+) and potassium (K+) ions moving in and out of cells. Without them you could not feel, move or think. That is why sports drinks contain "electrolytes" (a fancy name for ions).

Where do ions appear?

Ions appear in many situations:

  • In water: many substances split into ions when dissolved. Salt dissolves into Na+ and Cl- ions. Acids release H+ ions. Bases release OH- ions.
  • In batteries: a battery works by sending ions through a chemical inside it while electrons flow through the outside circuit. Lithium ion batteries (in phones and laptops) shuttle Li+ ions back and forth as they charge and discharge.
  • In your body: blood and cells are full of dissolved ions like Na+, K+, Ca2+, Mg2+, Cl- and HCO3- (hydrogen carbonate). They keep your heart beating, your nerves firing and your muscles contracting.
  • In the air after a thunderstorm: lightning ionises gas molecules in the atmosphere, briefly producing many ions (and creating the sharp smell of ozone).
  • In neon lights: when electricity flows through a tube of low-pressure neon gas, it knocks electrons off the neon atoms, turning them into ions. As the ions snap back together, they release coloured light.

Reading ion symbols

Ions are written with the element symbol and a small charge in the corner. The number says how many electrons were lost or gained. If no number is written, it means 1.

  • Na+: sodium ion, lost 1 electron.
  • Mg2+: magnesium ion, lost 2 electrons.
  • Al3+: aluminium ion, lost 3 electrons.
  • Cl-: chloride ion, gained 1 electron.
  • O2-: oxide ion, gained 2 electrons.

Some ions are made of several atoms that act as a unit, called polyatomic ions:

  • NH4+: ammonium
  • OH-: hydroxide
  • NO3-: nitrate
  • CO3 2-: carbonate
  • SO4 2-: sulfate
Did you know? Sea water contains about 35 grams of dissolved salts in every litre. Most of that is sodium chloride (Na+ and Cl-), but there are dozens of other ions too. There is so much dissolved magnesium that humans extract magnesium metal directly from sea water, and so much dissolved gold that the worlds oceans hold around 20 million tonnes of it (although too thinly spread to mine economically).

Ions and electricity

Pure water is a very poor conductor of electricity, but water containing dissolved ions conducts well. That is because the ions can move freely and carry electrical charge. This is why you should never use electrical devices near water: the dissolved minerals in tap water turn it into a conductor.

The same effect powers all batteries. Inside, ions move through a paste or liquid to balance the flow of electrons that goes through the outside wires to your device.

Try this Dissolve a teaspoon of salt in a cup of warm water and stir until clear. The crystals have broken into a sea of Na+ and Cl- ions floating freely. Now try to make a battery: stick a strip of copper and a strip of zinc-coated nail (or galvanised wire) into the salt water, not touching each other. Connect them through a small lightbulb or LED with wires. The ions in the salt water let a tiny current flow, lighting the bulb. You have just built a simple battery from a kitchen ingredient.
Deeper dive: how ions keep your heart beating

Every single heartbeat is controlled by a careful flow of ions. Heart muscle cells use four main ions: sodium (Na+), potassium (K+), calcium (Ca2+) and chloride (Cl-).

When the heart needs to beat, special "pacemaker" cells start by letting sodium ions rush into the cell. This makes the inside of the cell suddenly less negative compared to the outside. The change in charge spreads to the neighbouring cells like a wave, telling them to do the same. Then calcium ions rush in, which triggers the muscle fibres to contract. Finally, potassium ions flow back out, resetting the cell so it is ready for the next beat. The whole process takes about a third of a second and repeats roughly 100,000 times every day.

If the balance of ions gets disturbed, the heart can beat at the wrong rhythm. Too little potassium (after extreme vomiting or as a side effect of some medicines) can cause dangerous heart rhythm problems. Doctors monitor patients ion levels (especially K+, Na+ and Ca2+) very carefully in hospital. An ECG (electrocardiogram) is essentially a chart of ion flows in the heart, made visible as electrical signals on the skin.

So next time you feel your pulse, remember: every thump is millions of tiny ions hopping in and out of your cells in perfect rhythm.

For more, see chemical bonds and protons, neutrons and electrons.