In chemistry, an ion is a charged particle formed when an atom or group of atoms gains or loses electrons. Cations are positively charged ions formed when atoms lose one or more electrons. Anions are negatively charged ions formed when atoms gain one or more electrons. The terms come from Greek: 'cation' = 'going down' (attracted to cathode/negative electrode) and 'anion' = 'going up' (attracted to anode/positive electrode).
Cation: positively charged ion — formed by losing electrons.
Anion: negatively charged ion — formed by gaining electrons.
Metals form cations; non-metals form anions.
In electrolysis: cations move to cathode (-); anions move to anode (+).
Na⁺ (sodium cation): 11 protons, 10 electrons.
Cl⁻ (chloride anion): 17 protons, 18 electrons.
Ionic compound: cation + anion. Example: Na⁺ + Cl⁻ → NaCl.
Memory: CATion = POSITIVE; ANion = NEGATIVE.
Cation (positively charged ion):
Definition: An ion with a net positive charge, formed when an atom loses one or more electrons.
How cations form: • Metals tend to lose electrons to form cations • Atom → Cation + electrons • Na → Na⁺ + e⁻ (sodium loses 1 electron) • Mg → Mg²⁺ + 2e⁻ (magnesium loses 2 electrons) • Al → Al³⁺ + 3e⁻ (aluminium loses 3 electrons)
Property: Cations have MORE protons than electrons (protons > electrons)
Common cations: Cation | Name | Charge | Formed from H⁺ | Hydrogen / Proton | +1 | H atom loses e⁻ Na⁺ | Sodium | +1 | Na (Group 1) K⁺ | Potassium | +1 | K (Group 1) Ca²⁺ | Calcium | +2 | Ca (Group 2) Mg²⁺ | Magnesium | +2 | Mg (Group 2) Al³⁺ | Aluminium | +3 | Al (Group 13) Fe²⁺ | Iron(II)/Ferrous | +2 | Fe Fe³⁺ | Iron(III)/Ferric | +3 | Fe NH₄⁺ | Ammonium | +1 | N compound Cu²⁺ | Copper(II)/Cupric | +2 | Cu
In electrolysis: Cations move toward the cathode (negative electrode)
Anion (negatively charged ion):
Definition: An ion with a net negative charge, formed when an atom gains one or more electrons.
How anions form: • Non-metals tend to gain electrons to form anions • Atom + electrons → Anion • Cl + e⁻ → Cl⁻ (chlorine gains 1 electron) • O + 2e⁻ → O²⁻ (oxygen gains 2 electrons) • N + 3e⁻ → N³⁻ (nitrogen gains 3 electrons)
Property: Anions have MORE electrons than protons (electrons > protons)
Common anions: Anion | Name | Charge | Formed from Cl⁻ | Chloride | -1 | Cl (Group 17) Br⁻ | Bromide | -1 | Br (Group 17) I⁻ | Iodide | -1 | I (Group 17) F⁻ | Fluoride | -1 | F (Group 17) O²⁻ | Oxide | -2 | O (Group 16) S²⁻ | Sulphide | -2 | S (Group 16) N³⁻ | Nitride | -3 | N (Group 15) OH⁻ | Hydroxide | -1 | OH group NO₃⁻ | Nitrate | -1 | NO₃ group SO₄²⁻ | Sulphate | -2 | SO₄ group CO₃²⁻ | Carbonate | -2 | CO₃ group
In electrolysis: Anions move toward the anode (positive electrode)
Cation vs Anion — Key Differences:
Property | Cation | Anion Charge | Positive (+) | Negative (-) Formation | Loses electrons | Gains electrons Protons vs electrons | More protons | More electrons Typically formed by | Metals | Non-metals Movement in electrolysis | Toward cathode (-) | Toward anode (+) Example | Na⁺, Ca²⁺, Fe³⁺ | Cl⁻, O²⁻, SO₄²⁻
Memory trick: Cat-ion: CAT-ions are PAWSitively charged (Cats have paws, positive = paws = +) A-nion: An-ions are Negative
Or simply: • CATion: Cats are POSITIVE (friendly, positive charge) • ANion: AN-ions are NEGATIVE charges
Ionic compounds: When a cation and anion combine, they form an ionic compound: • Na⁺ + Cl⁻ → NaCl (sodium chloride / table salt) • Ca²⁺ + 2Cl⁻ → CaCl₂ (calcium chloride) • Mg²⁺ + O²⁻ → MgO (magnesium oxide)
In solution: • Ionic compounds dissociate into cations and anions in water • NaCl → Na⁺ + Cl⁻ (in water)
Cations are positively charged ions formed when atoms LOSE electrons (metals). Anions are negatively charged ions formed when atoms GAIN electrons (non-metals). In electrolysis, cations move toward the cathode (negative electrode) and anions move toward the anode (positive electrode). Examples: Na⁺ (cation), Cl⁻ (anion).
A cation is formed when a neutral atom loses one or more electrons. This gives the atom a net positive charge (more protons than electrons). Metals typically form cations. Example: Na → Na⁺ + e⁻ (sodium loses 1 electron to form Na⁺).
An anion is formed when a neutral atom gains one or more electrons. This gives the atom a net negative charge (more electrons than protons). Non-metals typically form anions. Example: Cl + e⁻ → Cl⁻ (chlorine gains 1 electron to form Cl⁻).
Cation examples: Na⁺ (sodium), K⁺ (potassium), Ca²⁺ (calcium), Mg²⁺ (magnesium), Fe²⁺/Fe³⁺ (iron), Al³⁺ (aluminium), NH₄⁺ (ammonium). Anion examples: Cl⁻ (chloride), F⁻ (fluoride), O²⁻ (oxide), S²⁻ (sulphide), OH⁻ (hydroxide), NO₃⁻ (nitrate), SO₄²⁻ (sulphate).
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