Chemical formulae and naming compoundsIB MYP Chemistry: Revision notes

Ionic compounds are made of ions: atoms (or groups of atoms) that have gained or lost electrons. A metal atom loses electrons to form a positive ion; a non-metal atom gains electrons to form a negative ion.

The charge can be read from the group of the periodic table:

• Group 1: 1+ (Na⁺, K⁺)
• Group 2: 2+ (Mg²⁺, Ca²⁺)
• Group 3: 3+ (Al³⁺)
• Group 6: 2− (O²⁻, S²⁻)
• Group 7: 1− (Cl⁻, Br⁻)

An ionic compound has no overall charge, so the positive and negative charges in its formula must cancel.

Use these steps:

1. Write the symbol and charge of each ion.
2. Find how many of each ion make the total charge zero (swap the charge numbers).
3. Write the numbers as subscripts and leave out any 1.

Worked example: magnesium chloride. Mg²⁺ and Cl⁻. One Mg²⁺ (2+) needs two Cl⁻ (2−), so the formula is MgCl₂.

Aluminium oxide: Al³⁺ and O²⁻. Two Al³⁺ (6+) balance three O²⁻ (6−), so the formula is Al₂O₃.

Always cancel to the simplest ratio: Mg²⁺ with O²⁻ is MgO, not Mg₂O₂.

A binary compound contains just two elements. To name an ionic one:

• Write the metal name first, unchanged: sodium, calcium.
• Change the end of the non-metal name to -ide: chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide.

Examples: NaCl is sodium chloride, MgO is magnesium oxide, K₂S is potassium sulfide.

Do not use number prefixes for ionic compounds. Al₂O₃ is aluminium oxide, not dialuminium trioxide.

A polyatomic ion is a group of atoms joined together with one overall charge. Learn these four:

• hydroxide OH⁻
• nitrate NO₃⁻
• sulfate SO₄²⁻
• carbonate CO₃²⁻

Name the compound with the metal first, then the ion name (no change to the ending): sodium hydroxide, calcium nitrate, magnesium sulfate, calcium carbonate.

When two or more of a polyatomic ion are needed, put it in brackets: calcium hydroxide is Ca(OH)₂ and aluminium sulfate is Al₂(SO₄)₃. Only use brackets when you need more than one of the group; sodium hydroxide is simply NaOH.

Covalent compounds are made of non-metal atoms that share electrons in molecules. Their names use prefixes to show how many atoms of each element are in one molecule:

• mono = 1, di = 2, tri = 3, tetra = 4

So carbon monoxide is CO, carbon dioxide is CO₂, sulfur dioxide is SO₂ and carbon tetrachloride is CCl₄. The first element drops 'mono' (it is carbon monoxide, not monocarbon monoxide).

Some common covalent compounds have everyday names you must learn: water H₂O, ammonia NH₃ and methane CH₄.

You can also work out a formula from how many bonds each atom makes: H makes 1, O makes 2, N makes 3 and C makes 4. In methane one carbon (4 bonds) joins four hydrogens (1 bond each), giving CH₄.

A covalent formula shows the actual atoms in one molecule; an ionic formula shows only the simplest ratio of ions.