Inside the atom
What is an atom actually made of?
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Explain
Atoms are made of even smaller particles. In the centre is a tiny nucleus, made of protons and neutrons.
Tiny electrons move around the outside. Protons are positive, electrons are negative, and neutrons have no charge.
The number of protons tells you which element it is. Change the sliders to see.
The atomic number Z is the number of protons. The mass number A = Z + N.
Neutral atoms have equal numbers of protons and electrons. Atoms with extra or missing electrons are called ions. Atoms of an element with different neutrons are isotopes.
Worked example: carbon-14 has 6 protons, so N = 14 − 6 = 8 neutrons.
- Protons and neutrons
- A tiny nucleus
- Electrons around it
Want the full story?
Electrons don't really orbit like planets. They are better pictured as clouds of probability. The nucleus is around 100,000 times smaller than the atom. Protons and neutrons are made of still smaller particles, called quarks.
The Formula
mass number = protons + neutrons
- the mass number, which has no unit
- the number of protons, called the atomic number, which has no unit
- the number of neutrons, which has no unit
Try the numbers
Mass number: 4 particles
Try it at home
You need your imagination, a large field or playground.
You need: your imagination, a large field or playground.
- Stand on a football pitch, or picture one.
- If the atom were as big as the pitch, the nucleus would be about the size of a pea or smaller.
- Everything else would be mostly empty space.
What's happening: an atom is mostly empty space, with almost all of its mass in the nucleus.
Physics is life
Your body
You are mostly empty space.
Daily life
Doctors use special atoms.
Nature
Old things can be dated by atoms.
Space
Atoms make up every star.
Quick check
Question 1 of 3
Words to know
Nucleus: The tiny, dense centre of an atom.