Inside the atom — the Bohr model
1.3 · The atom
Inside the atom — the Bohr model
By the end of this section:
- I can name the three particles in an atom and give the relative charge and mass of each
- I can describe and draw an atom using the Bohr model: a nucleus in the centre with electrons in shells around it
1 · Read
The notes, the key idea and the words to knowEverything around you is made of atoms. An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons that occupy shells (energy levels). The nucleus is unimaginably small but holds almost all of the atom’s mass; the rest of the atom is mostly empty space.
| Particle | Where it is | Relative charge | Relative mass |
|---|---|---|---|
| Proton | in the nucleus | +1 | 1 |
| Neutron | in the nucleus | 0 | 1 |
| Electron | in shells around it | −1 | almost 0 (about 1/1836) |
Why the atom looks the way it doesTap to open
Why is an atom neutral? It contains equal numbers of positive protons and negative electrons, so the two charges cancel out exactly. (This is the rule we lean on again and again: in a neutral atom, electrons = protons.)
Why do we say it is “mostly empty space”? In Ernest Rutherford’s famous gold-foil experiment, nearly all the tiny particles fired at a thin sheet of gold passed straight through; just a few bounced sharply back. The only way to explain that: almost all the mass and all the positive charge sit in a very small, dense nucleus, with the electrons far away and empty space in between.
Why is the Bohr model just a “model”? The Bohr model — electrons drawn in neat circles — is a simplification. Real electrons are not on tidy rings; they occupy fuzzy regions called orbitals, where we can only say where an electron is likely to be. We keep the Bohr model because it is simple and it still predicts how atoms behave. Our picture has improved each time new evidence arrived:
Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (nucleus) → Bohr (shells) → quantum (orbitals)
Reading a helium atom. Helium has 2 protons, 2 neutrons and 2 electrons. The 2 protons and 2 neutrons sit in the nucleus; the 2 electrons fill the first shell. Check the charge: 2 × (+1) from protons and 2 × (−1) from electrons gives 0 — neutral, exactly as a normal atom should be.
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Not thisElectrons sit inside the nucleus.
ThisOnly protons and neutrons are in the nucleus. Electrons are in shells around it.
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Not thisAn atom is a solid little ball.
ThisAn atom is mostly empty space — the nucleus is tiny compared with the whole atom.
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Not thisThe Bohr model is exactly how atoms are.
ThisIt is a model — useful but simplified. Real electrons occupy orbitals (probability clouds), not neat rings.
- atom
- the smallest particle of an element that still behaves like that element
- nucleus
- the tiny, dense centre of an atom; holds the protons and neutrons
- proton
- positive particle in the nucleus — relative charge +1, relative mass 1
- neutron
- neutral particle in the nucleus — relative charge 0, relative mass 1
- electron
- negative particle in the shells — relative charge −1, almost no mass
- shell (energy level)
- a region around the nucleus where electrons are found
- Bohr model
- a simple, useful model with electrons in fixed circular shells
- orbital
- the more accurate idea: a region where an electron is likely to be
2 · Watch
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We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.
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- Booklet pp. 11–14
- Knowledge Book pp. 11–14
- In your class workbook: Chapter 1.3 The ‘Bohr model’: Atoms, ions and isotopes (pp. 16–26) · Revise this section
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