Cation vs Anion: What's the Difference?

You've met ions — atoms with a charge. They come in exactly two flavours: cations and anions. The names sound almost identical, which is exactly why students mix them up on exams. Let's lock in the difference for good.

The short answer: a cation is a positively charged ion (an atom that lost electrons). An anion is a negatively charged ion (an atom that gained electrons). Cation = positive, anion = negative.

Quick comparison at a glance

Feature Cation Anion
Charge Positive (+) Negative (−)
How it forms Loses electrons Gains electrons
Protons vs electrons More protons than electrons More electrons than protons
Usually formed by Metals (+ NH₄⁺, H⁺) Nonmetals
Size vs the original atom Smaller than the atom Larger than the atom
Moves toward (in electrolysis) The cathode (−) The anode (+)
Examples Na⁺, Mg²⁺, Ca²⁺, NH₄⁺ Cl⁻, O²⁻, OH⁻, SO₄²⁻

Everything below is just these rows explained — including a couple of tricks so you never flip them again.

What is a cation?

A cation forms when an atom loses one or more electrons. With fewer negative electrons than positive protons, the leftover charge is positive.

Metals are the usual cation-makers, because they have only a few valence electrons and give them up easily:

  • Sodium loses 1 electron → Na⁺
  • Magnesium loses 2 → Mg²⁺
  • Aluminium loses 3 → Al³⁺

A cation is also smaller than the atom it came from — losing electrons often empties an entire outer shell, and the remaining electrons are pulled in tighter by the unchanged protons.

What is an anion?

An anion forms when an atom gains one or more electrons. With more electrons than protons, the overall charge is negative.

Nonmetals are the usual anion-makers, because they're only a few electrons short of a full shell:

  • Chlorine gains 1 electron → Cl⁻
  • Oxygen gains 2 → O²⁻

An anion is larger than its parent atom — the extra electrons add repulsion and swell the electron cloud.

How to remember which is which

Three tricks that actually stick:

  • "A negative ion" → a-n-ion → anion. The word anion literally hides "a negative ion."
  • Cats are paws-itive. A cation is positive.
  • The "t" in cation can be read as a plus sign: ca+ion.

Use whichever one sticks; you only need one.

A note on naming

Charges also change the name:

  • A cation usually keeps the element's name: Na⁺ is the "sodium ion."
  • A simple anion swaps its ending to -ide: Cl⁻ is "chloride," O²⁻ is "oxide."

That's why table salt (Na⁺ and Cl⁻) is called sodium chloride.

Worked examples

Classify each as a cation or anion:

  • Na⁺ → lost an electron, positive → cation.
  • Cl⁻ → gained an electron, negative → anion.
  • Ca²⁺ → lost two electrons → cation.
  • O²⁻ → gained two electrons → anion.
  • NH₄⁺ (ammonium) → positive polyatomic ion → cation.
  • SO₄²⁻ (sulfate) → negative polyatomic ion → anion.

Common mistakes to avoid

  • Flipping the signs. Cation = positive, anion = negative. Lean on one memory trick until it's automatic.
  • Thinking metals form anions. Metals lose electrons, so they form cations. Nonmetals form anions.
  • Assuming the ion is the same size as the atom. Cations shrink; anions grow.

FAQ

Is a cation positive or negative?
Positive. It formed by losing electrons, leaving more protons than electrons.

How do I remember cation vs anion?
"A negative ion" spells anion, and "cats are paws-itive" gives you the cation. Lock in one and the other is whatever's left.

Do nonmetals form cations or anions?
Anions — they gain electrons to fill their outer shell, picking up a negative charge.

Why is a cation smaller than its atom?
Losing electrons can remove a whole outer shell, and the unchanged protons pull the remaining electrons in more tightly.

The takeaway

Cations are positive ions made by losing electrons (usually metals); anions are negative ions made by gaining them (usually nonmetals). Remember "a negative ion = anion," recall that metals lose and nonmetals gain, and the two will never trip you up again.

Coming next → [What Is Electronegativity? Trends and Examples] — the atomic tug-of-war that decides who keeps the electrons. See also [What Is an Ion?] and [Ionic vs Covalent Bonds].

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