Elements & compounds, but bitesize.

Part 1What an element is

Everything around you is made of tiny particles called atoms. An atom is the smallest particle of a substance — far too small to see, even under a normal microscope. (If you want the details of what's inside an atom, that's the job of the atoms booklet — here we care about how atoms build up the materials around us.)

Some substances are made of only one type of atom. We call those substances elements. Gold is made only of gold atoms. Oxygen is made only of oxygen atoms. Carbon is made only of carbon atoms. An element can't be broken down into anything simpler by a chemical reaction — it's already as simple as it gets.

There are around 118 elements known to science, and every single one of them is laid out on a chart called the periodic table. About 90 of them occur naturally; the rest have been made in laboratories. Everything in the universe — you, the sea, the air, the stars — is built from this small set of building blocks.

Part 2Chemical symbols

Writing out "sodium" or "magnesium" every time would be slow, and not every chemist speaks English. So every element has a short chemical symbol — a code of one or two letters that scientists everywhere understand.

There's one rule you must never break. If the symbol has two letters, the first letter is ALWAYS a capital, and the second letter is ALWAYS lower-case. So carbon is C, calcium is Ca, and cobalt is Co. The capitals matter: get them wrong and you've written a different chemical entirely.

A few symbols look like they came from nowhere. That's because they come from the element's old Latin name, not its English one. Sodium is Na (from natrium), potassium is K (from kalium), iron is Fe (from ferrum), and lead is Pb (from plumbum — the same root as "plumber", who once worked with lead pipes).

Part 1Joining atoms together

An element is made of just one type of atom. But most of the substances you meet every day aren't pure elements — they're made of different atoms joined together. When atoms of two or more different elements chemically join, we get a compound.

"Chemically join" is the key phrase. The atoms aren't just sitting next to each other — they are bonded together, locked into a new substance with its own name and its own properties. Think of it like welding two pieces of metal: once they're welded, you can't just pull them apart with your hands.

Take water. Water is a compound. Each tiny particle of water is two hydrogen atoms bonded to one oxygen atom — that's why we write it H₂O. Hydrogen on its own is a flammable gas and oxygen on its own is a gas you breathe, but bonded together they make a liquid you can drink.

Part 2Splitting a compound up

Because the atoms in a compound are bonded together, you can't separate them by physical means — no amount of filtering, sieving, or picking would split water back into hydrogen and oxygen. The only way to break a compound apart is with a chemical reaction.

Here are three compounds you'll see again and again. Notice how each is made of different elements bonded together:

Part 1What the small numbers mean

A chemical formula tells you exactly which atoms a compound is made of, and how many of each. The symbols tell you which elements; the little numbers tell you how many.

Those little numbers are called subscripts, and the golden rule is: a subscript multiplies only the atom written just before it. If there's no number, it means "just one".

Take H₂O. The little 2 sits after the H, so it means two hydrogen atoms. The O has no number, so it means one oxygen atom. Total: 2 hydrogen + 1 oxygen = 3 atoms in all.

Part 2Counting atoms — step by step

To count the atoms in any formula, go along it left to right. For each symbol, read off its subscript (or count it as one if there's no number). Let's do a couple cleanly, the same way every time.

Part 3Brackets

Sometimes a formula has brackets with a number outside. The rule is just as tidy: the number outside the bracket multiplies everything inside the bracket.

Take Ca(OH)₂ — calcium hydroxide. The 2 outside the bracket applies to both the O and the H inside it. So there are two O atoms and two H atoms, plus the single Ca out front.

Part 1Metal + non-metal → -ide

The names of compounds aren't random — they follow rules, and once you know the rules you can name lots of compounds without being told.

The first rule: when a metal joins a non-metal, the metal keeps its name, and the non-metal's ending changes to -ide. So when sodium (a metal) joins chlorine (a non-metal), chlorine becomes chloride, and the compound is sodium chloride.

The same pattern works everywhere: magnesium + oxygen → magnesium oxide; iron + sulfur → iron sulfide; potassium + bromine → potassium bromide.

Part 2Two non-metals → prefixes

When two non-metals join, we sometimes need to say how many of each atom there are. We do that with prefixes — little words stuck on the front: mono- means one, di- means two, tri- means three.

That's why CO₂ is called carbon dioxide — "di" tells you there are two oxygen atoms. And CO is carbon monoxide — "mono" tells you there's just one. The prefix is doing the counting for you.

Part 3The -ate ending

Some compound names end in -ate instead of -ide. That ending is a clue: an -ate ending means oxygen is present in the compound, usually as part of a group of atoms alongside another element.

So copper sulfate contains copper, sulfur and oxygen — the "-ate" tells you the oxygen is in there. Calcium carbonate (the main compound in chalk and limestone) contains calcium, carbon and oxygen. Compare that with calcium sulfide, which is just calcium and sulfur — no oxygen, so it gets the plain -ide ending.

Part 1A brand-new substance

Here's one of the most surprising ideas in chemistry: a compound's properties are completely different from the properties of the elements it's made of. When elements bond, you don't get a blend of the two — you get something genuinely new.

The classic example is table salt, sodium chloride. Look at what it's made from:

· Sodium on its own is a soft, silvery metal so reactive it bursts into flame if you drop it in water.
· Chlorine on its own is a choking, poisonous green gas — it was once used as a weapon.

Bond them together and you get sodium chloride: a harmless white solid you sprinkle on your chips. An explosive metal and a poisonous gas combine to make something you can eat. That's how dramatically a compound can differ from its elements.

Part 2Chemical change vs physical change

Why are the properties so different? Because making a compound is a chemical change. In a chemical change, atoms are rearranged and new bonds form, so a brand-new substance is made — and it's very hard to reverse.

That's different from a physical change, like melting ice or dissolving sugar in tea. In a physical change no new substance is made — the molecules are the same, just rearranged or moved about — and it's usually easy to reverse (you can freeze the water again, or boil off the tea to get the sugar back).

Part 1What makes a mixture different

You've seen that a compound is chemically bonded. A mixture is the opposite kind of thing: it's just two or more substances mixed together but NOT chemically joined. The particles are mingled, but there are no bonds between the different substances.

Three differences are worth learning by heart:

· A compound is joined in a fixed ratio (water is always 2 hydrogen to 1 oxygen). A mixture can be in any ratio — you can make tea weak or strong.
· A compound is chemically bonded. A mixture is not bonded — the substances keep their own properties.
· A compound is hard to separate (needs a chemical reaction). A mixture is easy to separate by physical means like filtering, evaporating or using a magnet.

Part 2Everyday mixtures

Mixtures are everywhere. Two important examples:

Air is a mixture of gases — mostly nitrogen (about 78%) and oxygen (about 21%), with small amounts of others. The gases are mingled but not bonded, which is why we can separate them (factories chill air until the gases turn to liquid and boil off one by one).

Sea water is a mixture of water and dissolved salt (plus other bits). Because the salt is only mixed in, not bonded into the water, you can get it back easily: let the water evaporate and the salt is left behind. That's exactly how sea salt is made.