The Atom and Atomic Models Notes
AQA GCSE Physics: Revision notes
Key facts
- An atom has a tiny positive nucleus (protons and neutrons) surrounded by electrons.
- The radius of the nucleus is less than of the radius of the atom.
- Atomic number = protons; mass number = protons + neutrons.
- Isotopes have the same number of protons but different numbers of neutrons.
- Alpha scattering disproved the plum pudding model and led to the nuclear model.
Structure of the atom
An atom has a tiny, dense, positive nucleus of protons and neutrons, surrounded by electrons.
An atom has a tiny, positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons.
The radius of the nucleus is less than of the radius of the atom, so almost all of an atom's volume is empty space occupied by the electrons.
Lithium
2,1
Compared with the whole atom, the radius of the nucleus is:
Electron energy levels
Electrons occupy energy levels; absorbing radiation moves them up, and moving down emits radiation.
Electrons are arranged at different energy levels (shells) around the nucleus.
If an atom absorbs electromagnetic radiation, an electron can move to a higher energy level. When an electron moves to a lower energy level, electromagnetic radiation is emitted.
| Electron moves up | Electron moves down | |
|---|---|---|
| What happens | Moves to a higher energy level | Moves to a lower energy level |
| Electromagnetic radiation is | Absorbed | Emitted |
Electron moves up
- What happens:
- Moves to a higher energy level
- Electromagnetic radiation is:
- Absorbed
Electron moves down
- What happens:
- Moves to a lower energy level
- Electromagnetic radiation is:
- Emitted
What happens when an electron moves down to a lower energy level?
Atomic number and isotopes
Atomic number counts protons; mass number counts protons and neutrons; isotopes differ only in neutrons.
A neutral atom has equal numbers of protons and electrons, so it has no overall charge.
Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 and carbon-14 both have 6 protons, but carbon-14 has 8 neutrons.
If an atom loses outer electrons it becomes a positively charged ion.
Carbon-12
2,4
Carbon-14
2,4
- Atomic numbernumber of protons
- Mass numberprotons + neutrons
- Neutronsmass number − atomic number
Worked example
How many protons, neutrons and electrons are in a neutral atom of carbon-14 (atomic number 6)?
- 1
Protons = atomic number = 6.
- 2
Neutrons = mass number − atomic number = 14 − 6 = 8.
- 3
A neutral atom has equal protons and electrons, so electrons = 6.
An atom loses one outer electron. What is it now?
Development of the model
The model changed as new evidence emerged, and alpha scattering showed the atom has a tiny, dense nucleus.
In the alpha scattering experiment most alpha particles passed straight through gold foil, but a small number were deflected back.
This showed that the mass and positive charge are concentrated in a small, dense nucleus, which disproved the plum pudding model and led to the nuclear model.
Early 1800s
Dalton
Atoms as tiny solid spheres.
1897
Thomson
Discovers the electron: plum pudding model.
1909–11
Rutherford
Alpha scattering shows a tiny, dense nucleus.
1913
Bohr
Electrons in fixed energy levels.
1932
Chadwick
Discovers the neutron.
Solid sphere (Dalton)
Plum pudding (Thomson)
Nuclear model (Rutherford)
Bohr
2,1
Which observation from the alpha scattering experiment showed the plum pudding model was wrong?
Try an exam question
Describe how the results of the alpha scattering experiment led to the nuclear model of the atom.
[4 marks]
- [1]Most alpha particles passed straight through the gold foil.
- [1]So most of the atom is empty space.
- [1]A small number of alpha particles were deflected back.
- [1]So the mass and positive charge are concentrated in a small, dense nucleus, which disproved the plum pudding model.
That's the notes covered.
Carry on to the next subtopic.