AQA Physics 6.4.1.1
The structure of an atom
A tiny, dense, positive nucleus with electrons around it. Zoom in to scale, build an atom, then move an electron between energy levels.
Triple Science · Higher tier
Start here
A hand on a desk.
Press your hand flat on a desk. It stops at the surface and won’t go any further. Your hand and the desk are both made of atoms.
If you had to guess, is an atom mostly solid stuff, or mostly empty space?
Zoom · to scale
The nucleus is tiny. The rest is empty space.
An atom has a radius of about 1 × 10−10 m. The radius of its nucleus is less than 1/10 000 of that, around 1 × 10−14 m. Zoom in until you can see it.
The whole atom, radius about 1 × 10⁻¹⁰ m. At this scale the nucleus is far too small to draw: the cross marks where it is.
Picture it
If the nucleus were a football 30 cm across, the atom would be at least 3 km across.
Build an atom · not to scale
Protons and neutrons in the nucleus; electrons around it.
Almost all of the mass is in the nucleus. An atom has the same number of electrons as protons, so it has no overall charge. Task: make a neutral atom with 6 protons.
Overall charge +1: protons and electrons don’t balance, so this is an ion, not a neutral atom. Relative mass about 7.
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | nucleus |
| Neutron | 1 | 0 | nucleus |
| Electron | very small | −1 | energy levels around the nucleus |
Energy levels
Absorb radiation, move up. Emit radiation, move down.
Electrons are arranged at different distances from the nucleus: different energy levels. An electron moves further from the nucleus (to a higher energy level) only when it absorbs electromagnetic radiation, and moves closer (to a lower level) by emitting it.
The electron is at the lower energy level. It cannot move up on its own.
How many electrons each level holds is chemistry, in Metals and non-metals.
Think again · what is in the nucleus
“The nucleus is where the electrons are.”
Your call
ChooseWhich particles are in the nucleus of an atom?
Sort
Proton, neutron or electron?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Say what is where: nucleus, particles, energy levels, sizes.
- Explain
- Link cause and effect: equal numbers, so no overall charge.
- Compare
- Sizes in standard form: 10−10 m against 10−14 m.
Key fact
An atom has a positively charged nucleus of protons and neutrons, with electrons around it. The nucleus has less than 1/10 000 of the atom’s radius but almost all of its mass.
Key note · AQA 6.4.1.1
The structure of an atom
- Atoms are very small, with a radius of about 1 × 10⁻¹⁰ m.
- A positively charged nucleus of protons and neutrons, with electrons around it.
- The radius of the nucleus is less than 1/10 000 of the radius of the atom. Most of the mass is in the nucleus.
- Relative mass: proton 1, neutron 1, electron very small. Relative charge: +1, 0, −1.
- Electrons equal protons, so an atom has no overall charge.
- Electrons are at energy levels. Absorbing EM radiation moves one up; emitting moves it down.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure where the mass of an atom is?
The zoom figure keeps the nucleus-to-atom scale; the particles themselves, and the build and energy-level figures, are not to scale.