Lesson 1 of 3

Read and construct formulae

Subscripts count atoms; ion charges determine ratios.

SEC G2 Science K223 / K225 (2027)

I can use chemical symbols and formulae.Syllabus
Syllabus K223 / K225, 4.1(a). Use chemical symbols and formulae
I can translate atom counts and formulae.Syllabus
Syllabus K223 / K225, 4.1(b). Translate atom counts and formulae
I can balance ion charges to obtain formulae.Syllabus
Syllabus K223 / K225, 4.1(c). Balance ion charges to obtain formulae

Read and construct formulae

Subscripts count atoms; ion charges determine ratios.

An element symbol begins with a capital letter; a second letter is lower-case. Co is cobalt, while CO is carbon monoxide. In H₂SO₄, one formula unit contains 2 H, 1 S and 4 O atoms. Brackets multiply everything inside: Ca(OH)₂ contains 1 Ca, 2 O and 2 H. A coefficient multiplies the entire formula: 3H₂O represents six H atoms and three O atoms.

Worked exampleFrom atom counts to a formula

A model molecule contains two nitrogen atoms and four oxygen atoms. What is its formula, and how many oxygen atoms are in three molecules?

Try it first. Then show the worked stepsHide the worked steps

Write N and O with the atom counts as subscripts: N2O4.
Three molecules contain 3 x 4 = 12 oxygen atoms. Do not simplify N2O4 to NO2 when the actual atoms in one molecule are specified.

Answer

N₂O₄; 12 oxygen atoms in three molecules. A coefficient counts complete molecules; a subscript counts atoms within one.

A working symbol and formula bank
Family	Symbols or formulae
Common non-metals	H, C, N, O, F, P, S, Cl, Br, I; elemental gases H₂, N₂, O₂, Cl₂
Metals and other elements	Li, Na, K, Mg, Ca, Al, Zn, Fe, Cu, Ag, Pb; Si, He, Ne, Ar
Acids, alkalis and gases	HCl, HNO₃, H₂SO₄, NaOH, Ca(OH)₂, NH₃, CO, CO₂, SO₂, NO, NO₂, O₃
Common salts and reagents	NaCl, MgCl₂, CaCO₃, CuSO₄, AgNO₃, Ba(NO₃)₂, KI, KMnO₄
Oxides and organic examples	CaO, MgO, CuO, SiO₂; CH₄, C₂H₄, C₂H₅OH, CH₃COOH

An ionic formula must have zero total charge. Al³⁺ and O^2- balance at +6 and -6, giving Al₂O₃. Mg²⁺ and NO₃^- give Mg(NO₃)₂. Keep a polyatomic ion intact; use brackets only when more than one is needed.

Frequently used ion charges
Positive ions	Negative ions
Na⁺, K⁺, Ag⁺, NH₄⁺	Cl^-, Br^-, I^-, OH^-, NO₃^-
Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Pb²⁺	O^2-, CO₃^2-, SO₄^2-
Al³⁺, Fe³⁺	A Roman numeral gives a metal ion oxidation state; distinguish iron(II) from iron(III).

Check your understanding

A compound has the formula XCl3, and each chloride ion is Cl^-. What is the charge on X?

3+, because three Cl^- ions give -3 and the compound is neutral
3-, because the subscript 3 is the charge on the X ion
1+, because a metal ion in a chloride is like Na⁺ in NaCl

Show the answer

3+, because three Cl^- ions give -3 and the compound is neutral

Charges in an ionic compound add up to zero. Three Cl^- ions contribute -3, so the one X ion must be X³⁺.