Chemical Names and Formulas

What is it?

Every compound has a formula and a name. The formula shows which elements are present and in what ratio; the name says the same thing in words. $\ce{NaCl}$ is sodium chloride; $\ce{CO2}$ is carbon dioxide. Chemists around the world follow the same naming rules, set by IUPAC (the International Union of Pure and Applied Chemistry), so a name means exactly one substance.

There are two main families, named in different ways:

Ionic compounds, made of a metal (or the ammonium ion) combined with a non-metal: name the ions. Example: $\ce{MgCl2}$ , magnesium chloride.
Molecular (covalent) compounds, made of non-metals only: use Greek prefixes to count the atoms. Example: $\ce{N2O4}$ , dinitrogen tetroxide.

Key idea

An ionic compound has no overall charge: the positive charges of the cations exactly cancel the negative charges of the anions. That single rule lets you write the formula of any ionic compound from its name.

Why does it matter?

It’s the language of chemistry. Equations, labels, recipes and safety data sheets all use formulas and names.
Safety. Sodium chloride is table salt; sodium chlorate is a weedkiller and strong oxidizer. One syllable separates them.
Calculations depend on it. You can’t find a molar mass or balance an equation without the correct formula.

How does it work?

1. Common ions

You will meet these ions again and again:

Positive ions (cations)	Negative ions (anions)
$\ce{H+}$ hydrogen, $\ce{Na+}$ sodium, $\ce{K+}$ potassium, $\ce{Ag+}$ silver	$\ce{F-}$ fluoride, $\ce{Cl-}$ chloride, $\ce{Br-}$ bromide, $\ce{I-}$ iodide
$\ce{NH4+}$ ammonium	$\ce{OH-}$ hydroxide, $\ce{NO3-}$ nitrate, $\ce{HCO3-}$ hydrogencarbonate
$\ce{Mg^2+}$ magnesium, $\ce{Ca^2+}$ calcium, $\ce{Ba^2+}$ barium, $\ce{Zn^2+}$ zinc	$\ce{O^2-}$ oxide, $\ce{S^2-}$ sulfide
$\ce{Al^3+}$ aluminium	$\ce{SO4^2-}$ sulfate, $\ce{CO3^2-}$ carbonate
$\ce{Fe^2+}$ iron(II), $\ce{Fe^3+}$ iron(III)	$\ce{PO4^3-}$ phosphate, $\ce{N^3-}$ nitride
$\ce{Cu+}$ copper(I), $\ce{Cu^2+}$ copper(II)	$\ce{CH3COO-}$ ethanoate (acetate)

Groups of atoms that carry a charge together, such as $\ce{SO4^2-}$ , are polyatomic ions.

2. Naming ionic compounds

Name the cation first, then the anion: $\ce{KBr}$ is potassium bromide.
A simple anion from one element ends in -ide: chloride, oxide, sulfide.
Most polyatomic anions containing oxygen end in -ate: sulfate, nitrate, carbonate.
A metal that can form more than one ion (many transition metals) gets a Roman numeral for its charge: $\ce{FeCl2}$ is iron(II) chloride; $\ce{FeCl3}$ is iron(III) chloride.
No prefixes are used: $\ce{CaCl2}$ is calcium chloride, not calcium dichloride.

3. Writing ionic formulas

Write the two ions with their charges.
Choose the smallest numbers of each ion that make the total charge zero.
Write the numbers as subscripts (leave out 1). Put a polyatomic ion in brackets if you need more than one of it: $\ce{Ca(OH)2}$ , $\ce{(NH4)2SO4}$ .

4. Naming molecular compounds

Use a Greek prefix for the number of each kind of atom, and end the second element with -ide:

Number	Prefix	Number	Prefix
1	mono	6	hexa
2	di	7	hepta
3	tri	8	octa
4	tetra	9	nona
5	penta	10	deca

“Mono” is not used for the first element: $\ce{CO}$ is carbon monoxide, not monocarbon monoxide.
The final “a” or “o” of the prefix is dropped before “oxide”: monoxide, tetroxide, pentoxide.

Examples: $\ce{CO2}$ carbon dioxide, $\ce{SO3}$ sulfur trioxide, $\ce{N2O5}$ dinitrogen pentoxide, $\ce{SF6}$ sulfur hexafluoride. Some molecules keep common names: $\ce{H2O}$ is water, $\ce{NH3}$ is ammonia and $\ce{CH4}$ is methane.

5. Common acids

Acids are named by tradition: $\ce{HCl}$ hydrochloric acid, $\ce{HNO3}$ nitric acid, $\ce{H2SO4}$ sulfuric acid, $\ce{H3PO4}$ phosphoric acid, $\ce{H2CO3}$ carbonic acid and $\ce{CH3COOH}$ ethanoic (acetic) acid.

Think of it like this

Think of charges as weights on a see-saw that must balance exactly. To balance $\ce{Al^3+}$ (3 units on one side) against $\ce{O^2-}$ (2 units on the other), the smallest balanced load is two aluminium ions (6 units) against three oxide ions (6 units). So aluminium oxide is $\ce{Al2O3}$ .

More precisely

The Roman numeral is the oxidation state of the metal (see Redox Reactions). Older names used endings instead, such as ferrous ( $\ce{Fe^2+}$ ) and ferric ( $\ce{Fe^3+}$ ); you may still see them on old bottles. Hydrates include water in the crystal: $\ce{CuSO4.5H2O}$ is copper(II) sulfate pentahydrate. IUPAC prefers “hydrogencarbonate” to the older “bicarbonate”.

Visualise it

Flowchart. Does the compound contain a metal or the ammonium ion? If yes, it is ionic: name the cation, adding a Roman numeral if the metal forms more than one ion, then name the anion, ending in -ide or -ate; no prefixes; example FeCl3, iron(III) chloride. If no, it is molecular: use Greek prefixes for the number of atoms, never mono on the first element, and end in -ide; example N2O5, dinitrogen pentoxide. — Ionic or molecular? Decide first; the rules for naming follow.

Worked example

Worked example: Formula from a name: aluminium oxide

Question: Write the formula of aluminium oxide.

Ions: $\ce{Al^3+}$ and $\ce{O^2-}$ .
The smallest whole-number total that both 3 and 2 divide into is 6, so use 2 aluminium ions and 3 oxide ions:
$2 \times (+3) + 3 \times (-2) = 0$
Formula: $\ce{Al2O3}$ .

Worked example: A polyatomic ion: calcium nitrate

Question: Write the formula of calcium nitrate.

Ions: $\ce{Ca^2+}$ and $\ce{NO3-}$ . Two nitrate ions balance one calcium ion: $(+2) + 2 \times (-1) = 0$ .
Two nitrate ions need brackets: $\ce{Ca(NO3)2}$ . The subscript 2 applies to the whole $\ce{NO3}$ group, so the formula contains 1 Ca, 2 N and 6 O.

Worked example: Name from a formula: FeCl₃

Question: Name $\ce{FeCl3}$ .

Iron forms more than one ion, so find its charge. Three chloride ions give $3 \times (-1) = -3$ .
The compound is neutral, so iron is $+3$ : iron(III) chloride.

Worked example: Naming a molecular compound

Question: Name $\ce{P4O10}$ and write the formula of nitrogen trifluoride.

Two non-metals, so use prefixes: 4 = tetra, 10 = deca. $\ce{P4O10}$ is tetraphosphorus decoxide.
Nitrogen trifluoride: no prefix on nitrogen means 1, tri means 3: $\ce{NF3}$ .

Common mistake

Common mistake: Using prefixes for ionic compounds

$\ce{MgCl2}$ is magnesium chloride, not magnesium dichloride. The charges already fix the ratio, so ionic names never use prefixes. Prefixes are only for compounds of non-metals.

Common mistake: Mixing up -ide and -ate

Sodium sulfide is $\ce{Na2S}$ ; sodium sulfate is $\ce{Na2SO4}$ . “-ide” usually means a single element; “-ate” means a polyatomic ion with oxygen.

Common mistake: Forgetting brackets, or writing charges in the formula

Calcium hydroxide is $\ce{Ca(OH)2}$ , not $\ce{CaOH2}$ , which would mean one O and two H. And the charges are used to work out the formula but are not written in it: $\ce{NaCl}$ , not $\ce{Na+Cl-}$ .

Notation note

The Roman numeral follows the metal name directly, with no space: iron(III) chloride.
A charge is written as a superscript with the number first, then the sign: $\ce{Mg^2+}$ and $\ce{SO4^2-}$ .

Remember this

Metal (or $\ce{NH4+}$ ) + non-metal: ionic. Name the cation, then the anion; no prefixes.
Roman numerals give the charge of metals that form more than one ion: iron(II), iron(III), copper(II).
-ide: one element (chloride, oxide). -ate: polyatomic ion with oxygen (sulfate, nitrate).
Write ionic formulas so the charges add to zero; bracket a polyatomic ion used more than once.
Non-metals only: molecular. Use mono, di, tri, tetra…; no “mono” on the first element.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/chemical-names-and-formulas/