Stir a spoonful of salt into a glass of water, and the salt seems to vanish. But it is still there, in a form far too small to see: as tiny charged particles called ions.
Ions let salty water carry electricity. They carry messages along your nerves, and they travel back and forth inside the battery of a phone. Let’s find out what they are.
Atoms with too many or too few electrons
A normal atom has exactly as many electrons as protons. The protons have a plus electric charge, and the electrons have a minus charge. They cancel out, so the atom as a whole has no charge.
But electrons sit on the outside of the atom, and they can move from one atom to another. An atom that has lost or gained electrons is called an ion:
- An atom that loses electrons has more plus than minus. It becomes a positive ion.
- An atom that gains electrons has more minus than plus. It becomes a negative ion.
Scientists call positive ions cations and negative ions anions.
Only the electrons move. The protons stay locked in the nucleus, so the atom stays the same element. A sodium atom that loses an electron is still sodium, but now it is a sodium ion.
How do you write an ion?
Chemists write an ion as the element’s symbol with a small sign at the top right. The sign shows the charge: how many electrons are missing or extra.
| Ion | What happened | Its name |
|---|---|---|
| Na⁺ | a sodium atom lost 1 electron | sodium ion |
| Ca²⁺ | a calcium atom lost 2 electrons | calcium ion |
| Cl⁻ | a chlorine atom gained 1 electron | chloride ion |
| O²⁻ | an oxygen atom gained 2 electrons | oxide ion |
Metals, like sodium and calcium, usually lose electrons and become positive ions. Nonmetals, like chlorine and oxygen, usually gain electrons and become negative ions. The names of negative ions often end in -ide: chloride, oxide, fluoride.
Why do atoms give or take electrons?
It all comes down to the outer electron shell. Atoms are most settled when their outer shell is full.
A sodium atom has 11 electrons, arranged 2, 8, 1. Its single outer electron is easy to lose. Once it is gone, the full shell underneath becomes the outer shell: 2, 8.
A chlorine atom has 17 electrons, arranged 2, 8, 7. It needs only one more to fill its outer shell, so it grabs one whenever it can: 2, 8, 8.
When sodium meets chlorine, an electron jumps from the sodium to the chlorine in the same way. That gives a Na⁺ ion and a Cl⁻ ion, and plus and minus pull on each other. Countless plus and minus ions like these, packed together, make the white crystals of table salt. This hold between ions is one kind of chemical bond, called an ionic bond.
The lab below starts with a sodium atom. Take away one electron to make Na⁺, then try the other quests.
What happens when salt dissolves?
A salt crystal is a neat stack of Na⁺ and Cl⁻ ions, in a pattern like a three-dimensional chessboard. Look at salt through a magnifying glass: many of the grains are tiny cubes!
Drop salt into water, and the water crowds around the ions and pulls them out of the crystal, one by one. The ions spread out through the water. That is why the salt seems to disappear: it has broken up into ions far too small to see.
These ions can move. When two wires from a battery are dipped into salty water, the plus sodium ions drift towards the minus wire. The minus chloride ions drift towards the plus wire.
Moving charges make an electric current! So salty water carries electricity, while very pure water hardly does. Find out more in conductors and insulators.
Ions in your body
Your body is full of ions. Sodium, potassium, calcium and chloride ions float in your blood and inside your cells. Doctors call them electrolytes, and they have important jobs.
When you decide to wiggle a finger, a message races along your nerves. It travels as sodium ions rush into a nerve cell and potassium ions rush out again, a little further along each time.
It is a bit like a wave in a football stadium. Each fan only stands up and sits down, but the wave runs all the way round.
Calcium ions help your muscles squeeze, and that includes your heart. When you sweat a lot, you lose water and some of these ions too. That is why sports drinks talk about electrolytes.
Ions in batteries
Ions work hard inside batteries too. A phone or a tablet has a lithium-ion battery.
While the battery powers the phone, lithium ions travel inside it from one side to the other. At the same time, electrons flow around through the phone as an electric current. Charging pushes the lithium ions back again, ready for next time.
Go deeper: Charges, names and ions made of several atoms for grown-ups and the extra curious
- An ion’s charge is its number of protons minus its number of electrons, counted in units of the elementary charge, about coulombs. Na⁺ has 11 protons and 10 electrons.
- Michael Faraday named ions in 1834, from a Greek word meaning “going”, because they travel to the electrodes. In electrolysis, cations go to the cathode, the negative electrode, and anions to the anode, the positive one.
- Ions can be made of several atoms bonded together: hydroxide OH⁻, carbonate CO₃²⁻, sulfate SO₄²⁻ and ammonium NH₄⁺.
- H⁺ is a hydrogen atom without its only electron: a bare proton. In water it never stays alone. It attaches to a water molecule and forms H₃O⁺. Acids are substances that release H⁺ in water.
- About 99% of your body’s calcium is stored in your bones and teeth. The small rest, dissolved as Ca²⁺ ions, controls muscles and nerves.
Go deeper: Why doesn’t sodium give its electron away for free? for grown-ups and the extra curious
The sticker handover is a nice picture, but the energy tells a more surprising story. Pulling the outer electron off a sodium atom costs about 496 kilojoules per mole, which is 5.14 electronvolts per atom. A chlorine atom that catches an electron releases only about 349 kJ/mol. So turning a sodium atom and a chlorine atom into two separate ions actually costs energy overall!
The payoff comes afterwards. When the ions pack into a salt crystal, each one ends up surrounded by six neighbours of the opposite charge. Their pull releases about 787 kJ/mol. That big release is what makes salt form, and it is why salt crystals are so stable.
Go deeper: How much better does salty water conduct? for grown-ups and the extra curious
Very pure water conducts electricity extremely poorly: about siemens per metre. Only about two water molecules in every billion are split into ions, H₃O⁺ and OH⁻. Sea water holds about 35 grams of salt in every litre, and it conducts almost a million times better: about 5 siemens per metre.
At the wires, the ions don’t simply stop. They take or give electrons there, and new substances form. In salty water, bubbles of hydrogen appear at the minus wire, and chlorine can form at the plus wire. This is called electrolysis.
Check yourself
A calcium atom loses 2 electrons. What does it become?
Show the answer
Ca²⁺, a positive ion — Losing 2 electrons leaves it with 2 more protons than electrons, so its charge is plus 2. It is still calcium.
Losing 2 electrons leaves it with 2 more protons than electrons, so its charge is plus 2. It is still calcium.
Check yourself
Why does salty water carry electricity?
Show the answer
Salt breaks up into ions that can move through the water — In water, salt splits into Na⁺ and Cl⁻ ions. These charged particles can move, and moving charges make an electric current.
In water, salt splits into Na⁺ and Cl⁻ ions. These charged particles can move, and moving charges make an electric current.