Make a guess: why is salt safe to eat?
Watch, guess, then press Start.
Look around you — your chair, the air, even you. Everything is made of unimaginably tiny particles called atoms. They are far too small to see, even with a normal microscope.
An atom is the smallest particle of an element that can take part in a chemical change. There are only 118 known types of atom, one for each element, yet they combine to build every material in the universe.
Scale check: hundreds of thousands of atoms could line up across the width of a single human hair. Atoms really are that small.
Over 200 years ago the scientist John Dalton pictured atoms as tiny, solid spheres that cannot be broken down or destroyed. We call this the simple (Dalton) model of the atom.
Why it matters: because atoms cannot be created or destroyed, the atoms you start with must all still be there at the end of a reaction — just rearranged. That idea returns near the end of this lesson.
An element is a substance made from only one type of atom. You cannot break an element down into anything simpler by a chemical reaction.
There are around 118 known elements. They are all listed in the periodic table. Examples: oxygen, carbon, iron, gold, hydrogen and helium.
A compound is a substance made when atoms of two or more different elements are chemically joined (bonded) together, always in the same fixed ratio. It takes a chemical reaction to make a compound, and a chemical reaction to split it back up.
Compounds have completely different properties from the elements that make them. Sodium is a dangerous metal and chlorine is a poisonous, pale green gas — but bonded together they form sodium chloride: ordinary table salt, which is safe to eat in small amounts.
A mixture is two or more substances that are simply jumbled together but NOT chemically bonded. Because the substances are not bonded to each other, the parts of a mixture keep their own properties and can be separated physically (e.g. by filtering or evaporating).
Examples of mixtures: air (a mix of gases), sea water (salt mixed in water) and sand + iron filings.
Misconception alert: a compound is NOT a mixture. In a compound the atoms are chemically bonded and can only be split by a reaction. In a mixture the substances are not bonded to each other, so they can be separated physically.
Read each substance and tap whether it is an element, a compound or a mixture.
Methane burns in oxygen. At the end, the total mass of everything is…
Writing out element names is slow, so chemists give each element a short chemical symbol of one or two letters. It is a worldwide code, so scientists everywhere understand it.
The rule: the first letter is ALWAYS a capital; the second letter (if there is one) is ALWAYS lowercase. So it is Co for cobalt (a metal), but CO (capital C, capital O) means carbon-and-oxygen — a completely different thing!
Tap a symbol on the left, then tap the element it stands for on the right.
A chemical formula uses symbols to show which atoms — and how many of each — are in a substance. The small subscript number tells you how many of the atom just before it.
Symbol vs formula: a symbol stands for one element (like O or Na). A formula uses symbols and numbers to show the atoms in a substance, either an element (like O₂) or a compound (like H₂O or NaCl). Don't mix the two up!
In a chemical reaction, the atoms in the starting substances (the reactants) are rearranged — the old bonds break and new bonds form — to make new substances (the products).
Crucially, no atoms are created and no atoms are destroyed. Every atom you start with is still there at the end, just joined up differently.
Misconception alert: atoms are not made or lost in a reaction — they are only rearranged. This is exactly Dalton's idea that atoms cannot be created or destroyed.
Because the reaction just rearranges the same atoms, the total mass stays exactly the same. This is the conservation of mass:
If we know the masses of all-but-one of the substances, we can work out the missing one — because the total must balance.
Iron reacts with sulfur to make iron sulfide:
iron + sulfur → iron sulfide
28 g of iron reacts fully with 16 g of sulfur.
Mass of iron sulfide made = 28 + 16 = 44 g
The products weigh the same as the reactants because every atom is still there — none were created or destroyed.
Model answer: An element is made of only one type of atom and cannot be broken down chemically. A compound is made of two or more different types of atom chemically bonded together, and can only be split up by a chemical reaction.
Atom: the smallest particle of an element; Dalton pictured it as a tiny solid sphere that can't be created or destroyed.
Element: only one type of atom (e.g. O, Fe, Na).
Compound: atoms of two or more different elements chemically bonded in a fixed ratio (e.g. H₂O, CO₂, NaCl).
Mixture: substances jumbled together but not bonded — can be separated physically (e.g. air).
Symbol: one/two letters for an element (capital then lowercase). Formula: symbols + numbers showing the atoms in an element or a compound (e.g. O₂, H₂O).
Conservation of mass: reactants → products; atoms are rearranged, so total mass stays the same.
You've covered the key ideas in the KS3 topic — the Dalton model, atoms/elements/compounds/mixtures, symbols & formulae, and conservation of mass. Press Finish to see your score.
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