Atoms, elements and compounds

An element

HintGold, oxygen and iron are all examples — the simplest pure substances there are.

The whyAbout 118 of these are known, and each has its own square on the periodic table. Everything else in the universe is built by joining them together.

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.

The first letter

HintOnly one of the pair is upper case — the one you reach at the start.

The whyCo is the element cobalt, but CO is carbon monoxide, a compound of two elements. Case is part of the meaning of a symbol, not decoration.

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.