Atomic Structure and the Periodic Table
The Model of the Atom
AQA GCSE Chemistry
Models of the atom, in order
| Model | What led to it | The atom in this model |
|---|---|---|
| Tiny spheres | tiny spheres that could not be divided | |
| Plum pudding | the discovery of the electron | a ball of positive charge with (negative) electrons embedded in it |
| Nuclear model | the alpha particle scattering experiment | the mass of the atom is concentrated in the nucleus, the nucleus is positively charged, most of the atom is empty space, electrons orbit the nucleus |
| Bohr model | electrons orbit the nucleus at specific distances | |
| Protons | the positive charge of the nucleus is carried by protons | |
| Neutrons | Chadwick | Chadwick provided the evidence to show the existence of neutrons |
Order the particles were discovered: electrons, then protons, then neutrons.
Alpha particle scattering
| Observation | Conclusion |
|---|---|
| most alpha particles passed straight through the gold foil | most of the atom is empty space |
| some alpha particles were deflected | the atom has a positively charged nucleus |
- Also concluded: the mass of the atom is concentrated in the nucleus, and electrons orbit the nucleus.
- Shells came later, with Bohr. A conclusion from scattering is electrons orbit the nucleus, with no shells in it.
Plum pudding and nuclear model compared
| Feature | Plum pudding model | Nuclear model |
|---|---|---|
| Nucleus | no nucleus | has a nucleus |
| Positive charge | a ball of positive charge | the positive charge is all in the nucleus |
| Mass | the mass is spread out | the mass is concentrated in the nucleus |
| Empty space | no empty space | mostly empty space |
| Electrons | the electrons are embedded | the electrons and the nucleus are separate |
- Write each difference as a pair: what the plum pudding model has, then what the nuclear model has.
- Alike: both contain electrons and both are neutral overall.
- The model used today: a nucleus of protons and neutrons, with electrons in shells.
The plum pudding model came before the nucleus, the proton or the neutron was known, so its only particles are electrons. Describe it as a ball of positive charge with (negative) electrons embedded. Never write a ball of protons or a nucleus with electrons around it, and never put neutrons in the plum pudding. The same holds when it is set against the nuclear model: compare where the charge and the mass are, never the nuclear model has protons or the nuclear model has neutrons. Protons and neutrons belong only to the model used today.
Protons, neutrons and electrons
Protons and neutrons are in the nucleus; electrons are in the shells.
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | very small |
- Same relative mass: a neutron and a proton. Lowest mass: electron. No electrical charge: neutron.
- Protons and neutrons are in the nucleus and an electron's mass is very small, so almost all of the mass of an atom is in the nucleus.
- Atoms have no overall charge because they have the same number of protons and electrons.
- Atomic number is the number of protons. All atoms of one element have the same number of protons.
Size of atoms
| Radius | Size |
|---|---|
| Atom | about 0.1 nm, which is 1 × 10−10 m |
| Nucleus | less than 1/10 000 of the radius of the atom, about 1 × 10−14 m |
- 1 nm = 1 × 10−9 m.
- To compare two sizes, divide the larger radius by the smaller: the result is how many times larger.
- For a radius given as a fraction of another, multiply the larger radius by that fraction.
- A scale model keeps the same ratio: multiply the size of the model nucleus by the ratio of the atom to the nucleus.
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