Common Cations & Anions Master Chart Cheat Sheet

Key Facts

• A cation is a positively charged ion, such as sodium ion $\mathrm{Na^+}$, calcium ion $\mathrm{Ca^{2+}}$, or ammonium ion $\mathrm{NH_4^+}$.
• An anion is a negatively charged ion, such as chloride ion $\mathrm{Cl^-}$, oxide ion $\mathrm{O^{2-}}$, nitrate ion $\mathrm{NO_3^-}$, or sulfate ion $\mathrm{SO_4^{2-}}$.
• The total charge of an ionic compound must equal $0$, so $\text{total positive charge} + \text{total negative charge} = 0$.
• Group $1$ metals form $\mathrm{+1}$ cations, Group $2$ metals form $\mathrm{+2}$ cations, and aluminum commonly forms $\mathrm{Al^{3+}}$.
• Common monatomic anions include fluoride $\mathrm{F^-}$, chloride $\mathrm{Cl^-}$, bromide $\mathrm{Br^-}$, iodide $\mathrm{I^-}$, oxide $\mathrm{O^{2-}}$, sulfide $\mathrm{S^{2-}}$, and nitride $\mathrm{N^{3-}}$.
• Use the lowest whole-number ratio of ions when writing formulas, such as $\mathrm{Mg^{2+}}$ with $\mathrm{Cl^-}$ forming $\mathrm{MgCl_2}$.
• Use parentheses around a polyatomic ion when more than one is needed, such as $\mathrm{Ca(NO_3)_2}$ for calcium nitrate.
• A Roman numeral shows the charge of a variable-charge metal, so iron(III) means $\mathrm{Fe^{3+}}$ and copper(II) means $\mathrm{Cu^{2+}}$.

Vocabulary

Cation

A cation is an ion with a positive charge because it has lost one or more electrons.

Anion

An anion is an ion with a negative charge because it has gained one or more electrons.

Polyatomic ion

A polyatomic ion is a charged group of two or more bonded atoms that acts as one unit, such as $\mathrm{NO_3^-}$.

Ionic compound

An ionic compound is a neutral substance made of cations and anions held together by electrostatic attraction.

Subscript

A subscript is the small number in a chemical formula that shows how many atoms or ion groups are present, such as the $2$ in $\mathrm{MgCl_2}$.

Roman numeral

A Roman numeral in an ionic compound name shows the positive charge of a metal that can form more than one ion.