Everything is made
of atoms.
at the start of chemistry.
Cut, and cut, and cut again.
Take a square of gold leaf and tear it in half. Tear one half in half again. Keep going — half, and half, and half. For a long time you can carry on, and each piece is still gold. But there is a point where you cannot go any further without the gold stopping being gold. That smallest possible piece is an atom.
The Greeks guessed at this more than two thousand years ago — the word atom comes from a word meaning "uncuttable". They had no way to see one, so for centuries it stayed a hunch. Now we know it is real. An atom is staggeringly small: a line of about ten million of them, laid side by side, would stretch across a single millimetre. Everything around you — solid, liquid, gas — is built from these.
If an atom were blown up to the size of a football stadium, the heavy centre — the part that holds almost all of its mass — would be roughly the size of a pea sitting on the centre spot. The rest is mostly empty space, with the electrons whirling around the edge.
The atoms in your body are old — far older than you, far older than Earth. Almost every atom heavier than hydrogen was forged inside a star, then scattered when that star died. The carbon in your skin and the iron in your blood were made in stars that burned out long before the Sun was born.
Three particles, and a lot of empty space.
An atom is not the smallest thing after all — it is built from three smaller particles. At the centre sits a tiny, dense core called the nucleus. It holds two of the three particles: protons, which carry a positive charge, and neutrons, which carry no charge at all. The nucleus is almost all of the atom's mass, packed into almost none of its space.
Around the nucleus, far out, move the electrons — much lighter, each carrying a negative charge. They do not drift anywhere they like. They sit in layers called shells, each shell a fixed distance out. In a neutral atom the positives and the negatives balance exactly: the number of electrons equals the number of protons, so the charges cancel and the atom carries no overall charge.
In the nucleus
- Proton — positive charge (+). Heavy. Counting these tells you which element you have.
- Neutron — no charge (0). Heavy, about the same mass as a proton. Adds mass but not charge.
Around the nucleus
- Electron — negative charge (−). Very light. Sits in shells some distance out.
- Shells — the inner shell fills first (up to 2), then the next (up to 8), and so on.
A proton is about 1,836 times heavier than an electron. If a proton weighed as much as an adult, an electron would weigh about as much as a small grape. That is why we say the mass of an atom lives almost entirely in its nucleus.
Two numbers tell you almost everything.
Each atom carries two numbers, and between them they tell you what it is and how heavy it is. The first is the atomic number: the number of protons in the nucleus. This is the atom's identity card. An atom with one proton is hydrogen; with six, it is carbon; with seventy-nine, it is gold. Change the number of protons and you have changed the element itself.
The second is the mass number: the number of protons plus the number of neutrons added together — the count of the heavy particles in the nucleus. Electrons are so light they are left out of this sum. So if you know the atomic number and the mass number, you can work out everything: protons, neutrons, and (in a neutral atom) electrons too.
Take lithium, shown above as it appears on the periodic table. The small number is the atomic number, 3 — so the atom has 3 protons, and, being neutral, 3 electrons. The larger number is the mass number, 7. Neutrons are found by taking the mass number away from the atomic number: 7 − 3 = 4 neutrons. Three numbers, all from two.
protons = atomic number. neutrons = mass number − atomic number. In a neutral atom, electrons = protons.
Tap each card — what each particle does:
Gold is element number 79 — every gold atom, anywhere in the universe, has exactly 79 protons. That is what makes it gold and not lead (82) or platinum (78). For centuries alchemists tried to turn lead into gold by chemistry. It cannot be done that way: you would have to remove protons from the nucleus, which is a nuclear change, not a chemical one.
Elements, and what happens when they join.
An element is a substance made of just one kind of atom — every atom in it has the same number of protons. Pure gold is an element: nothing but gold atoms. Oxygen is an element; so is carbon, so is iron. There are about 118 elements known, and roughly ninety of them occur naturally on Earth. Everything else is built by combining them.
When atoms of different elements join and bond together, you get a compound. A compound is not a mixture — the atoms are chemically locked together, and the result behaves like a brand-new substance. Water is a compound: two hydrogen atoms bonded to one oxygen atom. Both hydrogen and oxygen are gases on their own, yet bond them and you get a liquid you can drink.
Element
- One kind of atom only.
- Cannot be broken into anything simpler by chemistry.
- Has its own square on the periodic table.
- Examples — gold (Au), oxygen (O), carbon (C), iron (Fe).
Compound
- Two or more elements chemically bonded.
- Has new properties, often unlike its parts.
- Can be split back into elements by a chemical change.
- Examples — water (H₂O), carbon dioxide (CO₂), salt (NaCl).
Take sodium, a soft silvery metal that bursts into flame in water, and chlorine, a choking green gas once used as a poison. Bond them and you get sodium chloride — ordinary table salt, which you sprinkle on your chips. The compound bears no resemblance to either element it is made from.
Only two elements are liquid at ordinary room temperature: mercury, the silvery metal in old thermometers, and bromine, a deep red-brown liquid. Every other element you meet is either a solid (like iron or carbon) or a gas (like oxygen or helium) when it is sitting in a normal room.
The table that puts the elements in their place.
For a long time the elements were just a list — dozens of substances with no obvious order. Then, in 1869, a Russian chemist named Dmitri Mendeleev laid them out on cards and shuffled them until a pattern appeared. He arranged them in order of mass, and noticed that certain properties came back round again and again — periodically. That is why we call it the periodic table.
Here is the part that made him famous. Mendeleev's pattern had gaps — places where the properties pointed to an element that belonged there, but no such element was yet known. Rather than force the table to fit, he left the gaps empty and predicted the missing elements: their mass, their colour, how they would behave. Within fifteen years three of them were found — including gallium and germanium — and they matched his predictions closely. A table that could foretell the unknown was a powerful thing.
The modern table keeps his idea but orders the elements by atomic number — by protons — rather than by mass, which sorts out a few oddities his version had. Reading it is straightforward once you know the two directions, which is next.
Columns, rows, and a line between metal and not.
The periodic table is a grid, and both directions mean something. The columns, running down, are called groups. Elements in the same group behave alike — that is the whole point of the arrangement. Group 1 elements are all soft, reactive metals; Group 0, the far right, are the unreactive "noble" gases. Reading down a group, you meet a family of elements with shared habits.
The rows, running across, are called periods. As you read across a period from left to right, you move through the elements one proton at a time, and their character shifts steadily — metal on the left, non-metal on the right.
Metals — left and centre
- Shiny when freshly cut.
- Good conductors of heat and electricity.
- Can be bent and pulled into shape.
- Most of the table — iron, copper, gold, sodium.
Non-metals — top right
- Often dull; brittle if solid.
- Poor conductors.
- Many are gases at room temperature.
- Oxygen, carbon, chlorine, helium.
Elements share a group because they have the same number of electrons in their outer shell. The outer shell is the part that does the reacting, so a shared outer arrangement means shared behaviour. That is the quiet reason the whole table works — it lines up atoms by how their outer electrons are set.
Atoms and elements, brought to life.
You've met the atom on paper — the nucleus, the shells, the idea that one kind of atom makes an element. Now watch it move. As you watch, try to hold onto two things: what makes one element different from another, and how atoms join to make compounds. Both are ideas you've just read.
Name the parts of the atom.
Each part of this atom is numbered. Pick a label below, then place it in the matching pin slot. Two of the labels don't belong inside an atom at all — read carefully before you place them.
Match each label to its pin
Count the protons.
How many protons does this atom have?
Fresh one. An atom has an atomic number of 11. How many protons does it have?
Work out the neutrons.
How many neutrons does it have?
Fresh one. An atom has an atomic number of 8 and a mass number of 16. How many neutrons does it have?
Which particle is positive?
Where does the mass sit?
Element, or compound?
What is an element?
Reading the table.
Mendeleev's bold move.
Explain the difference between an element and a compound.
Write a short answer in your own words. A strong one does three things: it says clearly what an element is, says what a compound is, and gives one example of each. If you can add a line on what happens when elements bond — that a compound can behave quite unlike the elements it came from — that lifts it further. Three or four sentences is plenty.
A few thoughts on your answer, Florence
strong You drew the line in the right place — an element being one kind of atom, a compound being different atoms bonded together. That distinction is the heart of the whole topic, and you put it plainly. Your example of water for the compound is a clean choice, the one most markers expect to see.
try this You named water as a compound but didn't quite name an element on its own — gold, or oxygen, would finish the pair. The question quietly asks for one of each, so make sure both halves get an example standing next to them.
to add One line on why the difference matters would carry this further: that a compound has new properties — sodium and chlorine, both dangerous alone, becoming harmless salt. A single "because" turns a description into an explanation.
Films and series for the world of the very small.
Sit down with Dad for any of these. They make atoms, elements and the people who chased them feel close. Heavier titles flagged for a chat first.
The words from today.
- Atom
- The smallest piece of an element that is still that element. Built from protons, neutrons and electrons.
- Nucleus
- The tiny, dense core at the centre of an atom, holding the protons and neutrons — and almost all the mass.
- Atomic number
- The number of protons in an atom. It is the element's identity — and decides its place on the periodic table.
- Mass number
- The number of protons plus neutrons in an atom. Neutrons = mass number − atomic number.
- Element
- A substance made of just one kind of atom. About 118 are known; each has its own square on the table.
- Compound
- A substance made of two or more elements chemically bonded together, with new properties of its own.
- Group / period
- A group is a column of the periodic table (elements that behave alike); a period is a row across it.
You've met the building blocks of everything.
You learned that everything is made of atoms, and what an atom is built from — protons, neutrons, electrons. You can find protons, neutrons and electrons from two numbers. You know an element from a compound, and you can read the periodic table that Mendeleev dared to leave gaps in. Florence, this is chemistry.
Film recommendations are factual reference only — see each title's own copyright owner.