Anion vs Cation: What's the Difference?
A cation is a positively charged ion formed when an atom loses one or more electrons, and an anion is a negatively charged ion formed when an atom gains electrons. That single difference, electrons lost versus electrons gained, is the whole of anion vs cation. Metals usually form cations, nonmetals usually form anions, and the opposite charges attract to build ionic compounds such as table salt. The names come from electrolysis: an anion travels toward the anode, and a cation travels toward the cathode.
Key takeaways
- A cation is positive because it holds fewer electrons than protons. An anion is negative because it holds more electrons than protons.
- Both are ions, meaning atoms or groups of atoms that carry a net electric charge.
- Atoms usually lose or gain electrons to reach the stable electron arrangement of the nearest noble gas.
- Metals form cations. Nonmetals form anions. The periodic table tells you which is which at a glance.
- A cation is smaller than the atom it came from, and an anion is larger.
- Ionic compounds are neutral overall, so charges must cancel: one Mg²⁺ needs two Cl⁻ ions to give MgCl₂.
What is a cation?
A cation is an ion with a positive charge, produced when a neutral atom loses one or more electrons. Protons stay locked in the nucleus during ordinary chemistry, so only electrons move.
Sodium is the standard example. A sodium atom has 11 protons and 11 electrons, so it is neutral. Lose one electron and it still has 11 protons but only 10 electrons, giving a net charge of 1+, written Na⁺. That ion now has the same electron arrangement as neon, which is why the change is so favorable. Magnesium gives up two electrons instead and becomes Mg²⁺.
Cations are always smaller than their parent atoms. Losing the outer electrons often empties the outermost shell completely, and the unchanged nuclear charge pulls the remaining electrons in more tightly.
What is an anion?
An anion is an ion with a negative charge, produced when a neutral atom gains one or more electrons. Adding negative particles to a balanced atom tips the total charge below zero.
Chlorine shows the pattern. A chlorine atom has 17 protons and 17 electrons. Gain one electron and it has 18 electrons against 17 protons, a net charge of 1−, written Cl⁻. Like Na⁺, it now matches a noble gas, in this case argon. Oxygen gains two electrons to become O²⁻.
Anions are larger than their parent atoms, for the mirror-image reason. The extra electron adds repulsion to a crowded outer shell without adding any protons to pull it back, so the electron cloud swells.
Watch the naming. A single-atom anion takes the ending -ide, so chlorine becomes chloride and oxygen becomes oxide. Cations simply keep the element name, as in sodium ion.
Anion vs cation: what are the key differences?
Every difference between the two follows from one fact: whether electrons were lost or gained.
| Cation | Anion | |
|---|---|---|
| Charge | Positive | Negative |
| How it forms | Atom loses electrons | Atom gains electrons |
| Electron count | Fewer electrons than protons | More electrons than protons |
| Typical elements | Metals | Nonmetals |
| Size vs parent atom | Smaller | Larger |
| Moves toward | The cathode | The anode |
| Naming | Keeps the element name | Single atoms end in -ide |
| Examples | Na⁺, Mg²⁺, Al³⁺, NH₄⁺ | Cl⁻, O²⁻, S²⁻, SO₄²⁻ |
How do you remember which is which?
Most students meet the same three memory aids, and it helps to know which one is trustworthy.
- The cat trick. A cat has paws, and a cation is “paw-sitive.” Silly, but it sticks.
- The letter t. Read the t in cation as a plus sign, so it becomes ca+ion.
- A Negative ION. The first two letters of anion can stand in for “a negative.”
The reliable version comes from where these words originated. Michael Faraday named these particles after the electrode each one travels toward when a current passes through a liquid. Anions head for the anode. Cations head for the cathode. Both words share a first letter with their destination, so the pairing is impossible to scramble.
Prefer that to any version phrased in plus and minus signs: the electrode signs are not fixed, but the direction of travel always is.
Which elements form cations, and which form anions?
The periodic table sorts this out for you. Elements on the left have few outer electrons and find it easier to hand them over. Elements on the right are only a step or two from a full shell, so they take electrons instead.
- Group 1 loses one electron: Na⁺, K⁺.
- Group 2 loses two: Mg²⁺, Ca²⁺.
- Group 13 commonly loses three: Al³⁺.
- Group 15 can gain three: N³⁻.
- Group 16 gains two: O²⁻, S²⁻.
- Group 17 gains one: F⁻, Cl⁻.
- Group 18 already has a full outer shell and forms ions only under extreme conditions.
Two groups of exceptions are worth knowing. Transition metals form more than one cation each, which is why iron appears as both Fe²⁺ and Fe³⁺ and copper as Cu⁺ and Cu²⁺. Hydrogen sits on the fence: it forms H⁺ in acids, yet it forms the hydride anion H⁻ with reactive metals.
Note the charge notation too: the number comes before the sign, so a magnesium ion is Mg²⁺ and never Mg⁺². A charge of one leaves the number out.
How do cations and anions combine into ionic compounds?
Opposite charges attract, and that attraction is ionic bonding. The resulting ionic compounds carry no overall charge, so the positive and negative charges inside them must cancel exactly.
Work through magnesium chloride. Magnesium forms Mg²⁺ and chlorine forms Cl⁻. One magnesium ion supplies 2+, and each chloride supplies 1−, so you need two chlorides to balance the books: (1 × 2+) + (2 × 1−) = 0. The formula is MgCl₂.
Aluminum oxide needs a bigger number. Al³⁺ and O²⁻ balance at six charges each way, since two aluminum ions give 6+ and three oxide ions give 6−, producing Al₂O₃. The shortcut is the crossover rule: each ion's charge number becomes the other ion's subscript. Always simplify afterwards, because Mg²⁺ with O²⁻ gives MgO rather than Mg₂O₂.
Many ions are polyatomic, meaning several atoms are bonded together and the group carries a charge as a unit. These behave like single ions in a formula.
| Name | Formula | Charge | Type |
|---|---|---|---|
| Ammonium | NH₄⁺ | 1+ | Cation |
| Hydroxide | OH⁻ | 1− | Anion |
| Nitrate | NO₃⁻ | 1− | Anion |
| Carbonate | CO₃²⁻ | 2− | Anion |
| Sulfate | SO₄²⁻ | 2− | Anion |
| Phosphate | PO₄³⁻ | 3− | Anion |
Ammonium is the common polyatomic cation; most of the others are anions. When a polyatomic ion needs a subscript, wrap it in parentheses first. Two ammonium ions with one sulfate ion give (NH₄)₂SO₄.
What happens to anions and cations in electrolysis?
Ions locked in a solid crystal cannot move. Melt that solid or dissolve it in water and the ions break free, turning the liquid into an electrolyte that conducts electricity. Pass a current through it and each type of ion heads for its own electrode.
Cations move to the cathode, where they collect electrons and are reduced. Anions move to the anode, where they give up electrons and are oxidized. Electrolysis of molten sodium chloride shows both halves clearly:
- At the cathode: Na⁺ + e⁻ → Na
- At the anode: 2Cl⁻ → Cl₂ + 2e⁻
One caution about signs. In an electrolysis cell the anode is the positive electrode and the cathode is negative, but in a battery those signs are reversed. The definitions themselves never change: oxidation happens at the anode, reduction happens at the cathode, and anions always travel toward the anode.
Frequently asked questions
Is a cation positive or negative?
A cation is always positive. It forms when an atom loses electrons, leaving more protons than electrons behind. A sodium cation, Na⁺, keeps its 11 protons but drops to 10 electrons, which leaves one unit of positive charge.
Why is a cation smaller than its parent atom?
Losing electrons frequently empties the entire outer shell, so the ion is built from fewer shells. The nucleus still has the same number of protons, so it grips the remaining electrons more tightly. An anion swells for the opposite reason.
Do anions and cations always pair up one to one?
No. The charges have to cancel, not the counts. Sodium and chloride happen to pair one to one in NaCl, but one Mg²⁺ needs two Cl⁻ ions, and two Al³⁺ ions need three O²⁻ ions. Always balance the total charge first.
Which comes first in a chemical formula, the cation or the anion?
The cation is written first, as in NaCl, MgCl₂, and Na₂SO₄. Names follow the same order, so you say sodium chloride rather than chloride sodium. Single-atom anions switch to an -ide ending in the name.
Want precise definitions of ion, electrolyte, and ionic bonding? Search any word in the A to Z Chemistry Dictionary.