4.1.3a The development of the model of the atom: early models
Models of the atom change when new evidence appears
Definition
Scientific model
A way of describing something we cannot see directly, used to explain observations and predict how it will behave.
- A scientific model is only kept while it fits the evidence.
- When an experiment gives results the old model cannot explain, the model is changed or replaced.
- The accepted picture of the atom has changed several times as new experiments were carried out.
The first model: tiny, solid, indivisible spheres
- Before the electron was discovered, atoms were thought to be tiny solid spheres that could not be divided into anything smaller.
- This model had no internal parts and no charged particles inside it.
The plum pudding model: electrons dotted through positive charge
Definition
Plum pudding model
An early model of the atom as a ball of positive charge with negative electrons embedded in it.
- The discovery of the electron showed that atoms do contain smaller, charged particles, so the solid-sphere idea could not be right.
- The plum pudding model pictured the atom as a ball of positive charge with tiny negative electrons dotted through it.
- In this model the positive charge was spread out evenly across the whole atom.
Note
- Picture a fruit scone: the dough is the spread-out positive charge and the raisins are the electrons set into it.
New evidence forces a new model: the nuclear atom
- To test the plum pudding model, a beam of positively charged alpha particles was fired at a very thin sheet of gold foil.
- Most of the alpha particles passed straight through the foil with little or no change in direction.
- A small number were deflected through large angles.
- A very few, about 111 in every 10 00010\,00010000, bounced almost straight back towards the source.
- Passing straight through shows that the atom is mostly empty space.
- The large deflections show there is a concentrated region of positive charge that repels the positively charged alpha particles.
- The few that bounce back show that this central charge is very dense and holds most of the atom's mass: this is the nucleus.
Key Idea
The scattering results showed that mass and positive charge are concentrated in a tiny central nucleus, so the nuclear model replaced the plum pudding model.
Common Mistake
- In the plum pudding model the positive charge is spread out with electrons embedded in it.
- In the nuclear model the positive charge and mass are packed into a tiny central nucleus, with electrons around the outside.
- The spread-out charge of the plum pudding model could not explain why some alpha particles bounced back, which is why the model was replaced.
Self review
- Why do scientific models of the atom get changed or replaced?
- What did the plum pudding model say the atom was made of?
- In the scattering experiment, what happened to most of the alpha particles?
- What did the small number of large deflections tell scientists about the atom?
- What did a few alpha particles bouncing back show about the nucleus?
4.1.3b The development of the model of the atom: the nuclear model develops
Bohr fixed the electrons into definite orbits
Definition
Bohr model
A refinement of the nuclear model in which electrons orbit the nucleus at specific, fixed distances.
- The nuclear model said electrons surround the nucleus, but it did not say where they were.
- Niels Bohr suggested that electrons orbit the nucleus at specific fixed distances, which are the energy levels.
- Bohr's theoretical calculations agreed with the results measured in experiments, which is why his idea was accepted.
Note
- These fixed distances are the same energy levels an electron moves between when it absorbs or emits electromagnetic radiation.
The positive charge of the nucleus splits into protons
Definition
Proton
A positively charged particle found in the nucleus; every proton carries the same size of positive charge.
- Later experiments showed the positive charge of a nucleus is not a single lump.
- It is made of a whole number of smaller particles, each carrying the same amount of positive charge.
- These particles were named protons.
Chadwick's work showed the nucleus also contains neutrons
Definition
Neutron
An uncharged particle found in the nucleus, with a mass close to that of a proton.
- Scientists still needed to account for the rest of the nucleus's mass.
- About 202020 years after the nucleus became an accepted idea, James Chadwick found evidence for a neutral particle in the nucleus.
- This particle is the neutron, which adds mass to the nucleus but carries no charge.
Key Idea
The model of the atom developed in stages:
- solid indivisible sphere,
- plum pudding model,
- nuclear model from the scattering experiment,
- Bohr's electron orbits,
- the positive charge split into protons,
- Chadwick's evidence for the neutron.
Note
- You are not expected to recall the experimental details of Bohr's calculations or Chadwick's work; focus on what each one concluded.
Self review
- How did Bohr change the nuclear model of the atom?
- Why was Bohr's model accepted by other scientists?
- What are the small particles of positive charge in the nucleus called?
- Whose work provided evidence for the neutron, and roughly how long after the nucleus was accepted?
- What does a neutron add to the nucleus, and what is its charge?
