Look around you: a wooden table, a glass of water, the air you breathe, your own hand. They could hardly be more different. Yet they are all built in the same way — from tiny building blocks called atoms.
What happens if you keep cutting?
Imagine a small piece of gold. Cut it in half. Then cut one of the halves in half again, and again, and again. Each new piece is smaller, but it is still gold.
Can you go on like this forever? No! In the end you reach the smallest bit of gold there is: a single gold atom. It is made of even smaller parts, but none of those parts is gold on its own.
No knife is sharp enough to really do this. But atoms are real. With special microscopes, scientists can even see them as tiny, blurry bumps.
How small is an atom?
Atoms are much smaller than anything you have ever seen. Not even a good school microscope can show them.
Look at one of your hairs and see how thin it is. To reach across it, you would need to line up several hundred thousand atoms side by side.
What is inside an atom?
An atom has a middle and an outside. The middle is called the nucleus. It is tiny, but it weighs almost as much as the whole atom.
The nucleus is made of two kinds of particles, packed tightly together:
- Protons carry a plus electric charge.
- Neutrons carry no charge at all. They are neutral, which is how they got their name.
Around the nucleus move much lighter particles called electrons. They carry a minus charge. Plus and minus pull on each other, and this pull keeps the electrons close to the nucleus. The electrons move in a fuzzy cloud, called the electron cloud.
A normal atom has exactly as many electrons as protons. The plus and the minus cancel out, so the atom as a whole has no charge.
Go deeper: A fuzzy cloud, not tiny planets for grown-ups and the extra curious
Many pictures show electrons circling the nucleus like planets around the Sun. Real electrons do not follow neat paths like that. Quantum physics can only tell where an electron is likely to be. These likely places form a fuzzy cloud around the nucleus, called an orbital. Orbitals come in different sizes and shapes, and they decide how an atom joins with other atoms.
Protons and neutrons are made of even smaller particles called quarks. Electrons, as far as anyone knows, are not made of anything smaller.
An atom is mostly empty
So why can’t your hand pass straight through a table? The electrons on the outside of your hand’s atoms push against the electrons of the table’s atoms. Minus pushes minus away, so your hand stops at the surface.
Go deeper: How small, exactly? for grown-ups and the extra curious
- Atoms are roughly 0.1 to 0.5 nanometres across. A nanometre is a millionth of a millimetre.
- A human hair is between about 0.02 and 0.18 millimetres thick. So several hundred thousand atoms fit across a typical hair.
- The nucleus is tens of thousands of times smaller across than the whole atom: about 27,000 times for uranium and about 60,000 times for hydrogen. Books often round this to “100,000 times”.
- Yet the nucleus holds more than 99.9% of the atom’s mass. A proton weighs about 1,836 times as much as an electron, and a neutron a little more.
- What really stops your hand at the table is the electric push between electrons, together with a quantum rule called the Pauli exclusion principle. It stops electrons from crowding into the same state.
How many kinds of atoms are there?
Gold atoms, oxygen atoms, iron atoms… Scientists have found 118 different kinds of atoms so far. Each kind is called a chemical element.
What makes one element different from another? The number of protons in the nucleus. Every hydrogen atom has 1 proton, every carbon atom has 6, every oxygen atom has 8 and every gold atom has 79.
More than 90 elements are found in nature. The others were made by scientists in laboratories. All of them are listed in a famous chart called the periodic table.
What do atoms do?
Atoms often hold on to each other. A group of atoms stuck together is called a molecule. The smallest bit of water, for example, is a molecule made of two hydrogen atoms and one oxygen atom.
Most things around you are built from only a handful of kinds of atoms, joined in countless ways. It is like building a whole town from a box that holds only a few kinds of bricks.
Atoms never sit still, either. They jiggle and bump into each other all the time. The warmer something is, the faster its atoms jiggle — and that is what temperature tells us.
Check yourself
What makes a gold atom different from an oxygen atom?
Show the answer
The number of protons in its nucleus — Every element has its own number of protons. A gold atom always has 79, and an oxygen atom always has 8.
Every element has its own number of protons. A gold atom always has 79, and an oxygen atom always has 8.