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A sports drink advertises “electrolytes”. A water filter promises to remove “heavy metal ions”. A hair dryer claims to use “negative ions”. All of them are talking about the same thing: atoms or groups of atoms that carry an electric charge.
The definition
An atom is electrically neutral: it has the same number of protons (positive) as electrons (negative). An ion is an atom or group of atoms that has gained or lost electrons, so the charges no longer balance.
- Lose electrons → more protons than electrons → positive ion, called a cation.
- Gain electrons → more electrons than protons → negative ion, called an anion.
Only electrons change. The number of protons — the atomic number, which makes an element what it is — never changes in ordinary chemistry. A sodium ion is still sodium.
A mnemonic: ca+ions are paws-itive (cat paws); anions are negative. Or: anion is negative.
Writing ions
The charge is written as a superscript after the symbol, number first then sign:
- Na⁺ (lost 1 electron)
- Mg²⁺ (lost 2)
- Al³⁺ (lost 3)
- Cl⁻ (gained 1)
- O²⁻ (gained 2)
- N³⁻ (gained 3)
Why atoms form ions
Atoms tend to gain or lose electrons until they reach the same electron arrangement as the nearest noble gas, which is especially stable (see electron configuration rules).
- Sodium (2, 8, 1) loses its one outer electron → Na⁺ (2, 8), like neon.
- Chlorine (2, 8, 7) gains one electron → Cl⁻ (2, 8, 8), like argon.
Metals, with few outer electrons and low ionization energies, tend to lose electrons and form cations. Nonmetals, with nearly full outer shells, tend to gain electrons and form anions.
Predicting charge from the periodic table
For main-group elements, the group number tells you the usual ion charge:
| Group | Typical ion | Examples |
|---|---|---|
| 1 | +1 | Li⁺, Na⁺, K⁺ |
| 2 | +2 | Mg²⁺, Ca²⁺ |
| 13 | +3 | Al³⁺ |
| 15 | −3 | N³⁻, P³⁻ |
| 16 | −2 | O²⁻, S²⁻ |
| 17 | −1 | F⁻, Cl⁻, Br⁻ |
| 18 | none | noble gases rarely form ions |
Group 14 elements like carbon and silicon usually share electrons instead of forming simple ions.
Transition metals are less predictable: they often form more than one ion. Iron forms Fe²⁺ and Fe³⁺; copper forms Cu⁺ and Cu²⁺. That’s why their compound names include Roman numerals — see how to name ionic compounds. Each element page on this site lists the element’s known oxidation states.
Size changes too
- Cations are smaller than their atoms. Losing the outer shell and having more protons than electrons pulls the rest in tight. Na⁺ is roughly half the radius of an Na atom.
- Anions are larger than their atoms. Extra electrons repel each other and spread out. Cl⁻ is noticeably larger than a Cl atom.
Polyatomic ions
Some ions are made of several atoms bonded together, carrying one overall charge: sulfate SO₄²⁻, nitrate NO₃⁻, ammonium NH₄⁺, hydroxide OH⁻. They stay together as units in compounds. See our polyatomic ions list.
Ions in action
- Ionic compounds like table salt are made of cations and anions held together by electrostatic attraction. See ionic vs. covalent bonds.
- Electrolytes are ions in solution that let it conduct electricity. Pure water barely conducts; salt water conducts well.
- Your nervous system runs on ions. Nerve impulses are waves of Na⁺ and K⁺ ions moving across cell membranes, and your heartbeat depends on Ca²⁺ as well.
- Batteries work by moving ions — in a lithium-ion battery, Li⁺ ions shuttle between the electrodes as it charges and discharges.
- Hard water contains dissolved Ca²⁺ and Mg²⁺.
Ions vs. isotopes
Both are “versions” of an element, which is why they get confused. Ions differ in their number of electrons; isotopes differ in their number of neutrons. We untangle them in isotopes vs. ions vs. isomers vs. allotropes.
Quick answers
How do you find the number of electrons in an ion? Atomic number minus the charge. Mg²⁺: 12 − 2 = 10 electrons. Cl⁻: 17 − (−1) = 18. See how to find protons, neutrons and electrons.
Can an ion gain or lose protons? Not in chemical reactions — only in nuclear reactions, which turn it into a different element.
Are ions dangerous? Not in general. Many are essential nutrients (Na⁺, K⁺, Ca²⁺, Fe²⁺). Some, like Pb²⁺ and Hg²⁺, are toxic.
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