Atoms, elements and compounds

Diatomic

HintSeven elements behave this way in their ordinary form, hydrogen and chlorine among them.

The whyHydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine all exist as joined pairs rather than lone atoms, which is why the gas in the air is written O2 and never O. Writing a single symbol where a pair belongs is the commonest slip in a KS3 symbol equation, and it is what makes such an equation refuse to balance.

A molecule

HintOxygen gas travels about in pairs and water in threes — what is each of those joined-up units?

The whyWritten as formulae, oxygen gas is O2 and water is H2O. The atoms in each are held together firmly enough that the group travels as one particle.

Na

HintSeveral metals take their symbol from an older Latin name rather than from the English one.

The whyThe symbol comes from natrium, the Latin name. Symbols are identical in every language, so a formula written in Beijing reads the same in Bristol.

It holds carbon, oxygen and chlorine, but no cobalt

HintEvery capital starts a new symbol, so count how many separate elements each formula actually names.

The whySplit at each capital and CoCl2 gives Co then Cl2 — one metal atom with two chlorine atoms, the pink salt used in humidity indicators. COCl2 splits into C, O and Cl2: three non-metals, and the substance is phosgene, a highly poisonous industrial gas. Six identical characters, one shift key apart, name two utterly different things.

A compound

HintWater is one, but air is not — the parts are locked together rather than simply sharing the same space.

The whyBecause nothing is chemically joined in a mixture, filtering or evaporating can separate it. Splitting the joined kind needs a chemical reaction.

Table salt

HintYou shake it over your chips at teatime.

The whySodium chloride behaves nothing like either ingredient. Joining atoms chemically produces a genuinely new substance, so you cannot predict a compound from the elements that made it.

A physical change

HintNothing new appears, and a spell in the freezer takes you straight back to where you started.

The whyMelted ice is still water, so no new substance has been made and cooling reverses it. A chemical reaction produces different substances that cannot simply be cooled back.

There are two oxygen atoms

HintThe little number counts only the symbol written immediately before it.

The whyA subscript belongs to the symbol directly in front of it, so CO2 is one carbon with a pair attached. Where no small number is written, there is exactly one.

Oxygen

HintCompare sulfide with sulfate — the longer ending adds the gas that makes up a fifth of the air.

The whyCopper sulfide contains only copper and sulfur; copper sulfate contains copper, sulfur and oxygen. An -ide ending signals a compound of just two elements.

Five

HintEverything inside the bracket is doubled, then the calcium outside counts once.

The whyThe 2 outside the bracket applies to both symbols inside, giving two oxygen and two hydrogen atoms, plus the single calcium.

chain

HintPicture a paper garland: the same link added again and again until it stretches across the room.

The whyThe repeating unit is called a monomer, and poly- means many. It is the enormous length of these molecules that gives plastics their strength and flexibility.

MgO

HintOne atom of each, with the metal written first and no small numbers needed.

The whyMagnesium and oxygen combine one to one, so no subscripts appear. The metal always comes first, and the -ide ending confirms only two elements are present.