1.2 Atomic structure and classification of matter

done

Taught 8 Sep – 9 Sep · Booklet pp. 8–15 · Homework week 2

By the end of this, you should be able to:

Classify matter into groups: mixtures, compound substances, elemental substances
Represent pure substances (compound and elements) using an ‘atom’ symbol or formula (e.g. Au, H2O)
Appreciate that the atoms of different elements can be represented by a symbol, which is shown in

This chapter also builds on 1.1 Substances, states of matter and the particle model — those notes are not repeated here.

The shorthand of chemistry

What you need to understand

Chemists write substances in a shorthand that is the same in every language. Each element has a chemical symbol of one or two letters — the first always a capital, the second (if there is one) always lowercase: C, He, Na, Fe. Compounds are written as a chemical formula: symbols with subscripts that show how many of each atom are present.

Reading a formula

- A subscript applies only to the symbol immediately before it: H₂O = 2 hydrogen, 1 oxygen.

- Brackets group atoms; a subscript outside the bracket multiplies everything inside: Ca(OH)₂.

- A big number in front (a coefficient) multiplies the whole formula: 2 CO₂ = 2 carbon, 4 oxygen.

Why the rules are written so strictly

Why it works

The capital-then-lowercase rule is not fussiness — it removes ambiguity. Co is the element cobalt, but CO is the compound carbon monoxide (one carbon, one oxygen). One letter case changes the meaning entirely, so the convention has to be exact.

How many atoms of each element are in Ca(OH)₂?

The bracket (OH) is multiplied by 2 → 1 calcium, 2 oxygen, 2 hydrogen — 5 atoms in total.

How many oxygen atoms are in 2 H₂SO₄?

One H₂SO₄ has 4 oxygen. The coefficient 2 multiplies the whole formula → 8 oxygen atoms (and 4 hydrogen, 2 sulphur).

Going deeper

Some symbols look nothing like the English name because they come from Latin: Na (sodium) from natrium, Fe (iron) from ferrum, Pb (lead) from plumbum — the root of the word "plumber". This shared shorthand means a chemist anywhere in the world reads H₂SO₄ the same way, which is exactly why science can be checked and built on across borders. Next you will use these formulae inside equations, where the counting you practised here becomes the basis of balancing and the conservation of mass.

Diagrams and worked examples are in the Knowledge Book PDF below — they don't reproduce well on a page.

Now test yourself

Say it out loud before you tap.

Words you need

chemical symbol define it

one or two letters for an element; first letter capital, second lowercase

E.G.C, Ca, Cl, Na, Fe

NOTCO — that is a formula for a compound; and NA or nA — wrong case

chemical formula define it

symbols and subscripts showing the atoms in a substance

E.G.H₂SO₄ — 2 H, 1 S, 4 O

NOT"sulphuric acid" — that is the name, not the formula

subscript define it

the small number after a symbol; counts the atoms of that element only

E.G.the 2 in H₂O counts hydrogen only → 2 H and 1 O

NOTthe 2 in 2 H₂O — that one is a coefficient and multiplies everything

coefficient define it

the large number in front of a formula; multiplies the whole formula

E.G.2 H₂O means 4 H and 2 O in total

NOTthe small 2 inside H₂O — never change that

diatomic molecule define it

a molecule of two atoms, e.g. O₂, H₂, Cl₂

E.G.H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

NOTO₃ (three atoms), He (a single atom), H₂O (two different elements)

Easy to get wrong

TRAP CO and Co mean the same thing. true or false?

TRUTH No — CO is carbon monoxide (two atoms); Co is the element cobalt. Letter case is part of the meaning.

TRAP A subscript can multiply a whole formula. true or false?

TRUTH A subscript only counts the symbol before it. To multiply the whole formula, use a coefficient in front (2 H₂O).

TRAP 2 H₂O has 2 oxygen atoms in each molecule. true or false?

TRUTH Each water molecule still has 1 oxygen. The 2 means two molecules, so 2 oxygen atoms in total.

TRAP You can change a subscript to balance an equation. true or false?

TRUTH Never — changing a subscript changes the substance. You only adjust coefficients (see 1.7).

Check yourself

Atoms in Ca(OH)₂? A. 3 B. 4 C. 5 D. 6 answer

Try this

PhET Build a Molecule opens in a new tab Story The search for the atomHow we worked out what atoms are, and how anyone knows. Finish it for the Story Complete badge. opens in a new tab

Test yourself

Build a formula

Classify substances

Formulae, symbols & counting atoms

Formulae ladder early

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