C1 Atomic structure: History of the atom
Embedded Files
Our understanding of the atom has evolved over time as scientists have conducted experiments and refined their theories.
New experimental evidence may lead to a scientific model being changed or replaced.
Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
John Dalton --> indivisible sphere model
In the 1800s John Dalton believed that atoms are tiny spheres that could not be broken down.
Atoms re-arrange themselves and combine with other atoms.
The discovery of the electron led to the plum pudding model of the atom.
JJ Thompson --> plum pudding model
In the 1890s JJ Thompson proposed the plum pudding model:
the atom is a ball of positive charge with negative electrons embedded in it.
Ernst Rutherford --> the nuclear model
1909 - 1913 Ernst Rutherford designed an experiment carried out by Geiger and Marsden – they fired positively charged alpha particles at a piece of very thin gold foil (only a few atoms thick) and measured the different paths taken by the alpha particles.
Here are some of the observations and conclusions reached from the experiment:
Some alpha particles are deflected because the mass of the atom is concentrated at the centre of the atom, the nucleus was positively charged.
Most alpha particles were not deflected because the atom is mainly empty space.
This nuclear model replaced the plum pudding model.
Niels Bohr --> electron shells
In 1914 Niels Bohr adapted the nuclear model by suggesting that:
electrons orbit the nucleus at specific distances in certain fixed energy levels (shells).
The theoretical calculations of Bohr agreed with experimental observations.
Later experiments led to the idea that:
the positive charge of any nucleus could be subdivided into a whole number of smaller particles,
each particle having the same amount of positive charge.
The name proton was given to these particles.
The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus.
This was about 20 years after the nucleus became an accepted scientific idea.
Q1. What was the earliest model of the atom?
Dalton’s model - the atoms are tiny spheres that could not be broken down.
Q2. The diagram below shows one of the earliest model of the atom.
Who proposed this model?
John Dalton
Q3. The diagram below also shows one of the earliest model of the atom.
Who proposed this model?
JJ Thompson
Q4. What is the name of the model proposed in the diagram above?
The plum pudding model.
Q5. Describe the plum pudding model of the atom?
A ball of positive charge with negative electrons embedded into it.
Q6. Which sub-atomic particle did JJ Thomson discover?
The electron
Q7. The diagram below shows Rutherford’s experiment.
Briefly describe Rutherford’s experiment.
Alpha particle scattering – Rutherford fired positively charged alpha particles at a piece of very thin gold foil.
Q8. Why did most alpha particles pass straight through the gold foil?
Because most of the atom is empty space.
Q9. Why were some alpha particles deflected / reflected?
Because:
- the mass of the atom is concentrated in the nucleus / centre
- the atom has a positively charged nucleus / centre.
Q10. What did the gold foil experiment (alpha particle scattering) prove?
The mass of the atoms is concentrated in a positively charged nucleus, with orbiting electrons.
Q11. Describe how Bohr revised Rutherford’s model of the atom.
Bohr suggested electrons orbit the nucleus at specific distances in certain fixed energy levels (shells).
Q12. Name the scientist who discovered the neutrons.
James Chadwick
Q13. In the modern atomic model, how are the sub-atomic particles arranged in an atom?
Protons and neutrons in the nucleus, electrons orbiting the nucleus on electron shells (energy levels).
Page updated
Google Sites
Report abuse