The History of the Atom Flashcards
What are atoms?
Tiny particles of matter (stuff that has a mass) which makes up everything in the universe.
At the start of the 19th century, John Dalton described atoms as what?
Solid spheres, and said that different spheres made up different elements.
In 1897, who concluded that atoms weren’t solid spheres?
J.J Thomson.
How did J.J Thomson conclude that atoms weren’t solid spheres?
His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles - electrons.
What model did J.J Thomson use to show his theory?
The plum pudding model, with a positively charged ‘pudding’ and negatively charged electrons.
In 1909, who showed that the plum pudding model was wrong?
Ernest Rutherford, with his students Hans Geiger and Ernest Marsden.
What experiment did Rutherford conduct to prove that the plum pudding model was wrong?
The gold foil experiment.
What was the gold foil experiment?
Firing positively charged alpha particles at an extremely thin sheet of gold.
What was Rutherford expecting to see from the plum pudding model?
The particles were expected to pass straight through the sheet or be slightly deflected at most.
Why was Rutherford expecting to see the particles pass through the gold sheet?
Because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom.
What did Rutherford actually see when the particles were fired at the gold sheet?
Most of the particles did go through the sheet, some were deflected more than expected, and a small number were deflected backwards.
What did Rutherford conclude from what he saw?
That there is a tiny, positively charged nucleus at the centre of the atom, surrounded by a ‘cloud’ of negative electrons.
What did Niels Bohr suggest should be changed from Rutherford’s model?
1) The electrons are all in shells.
2) The electrons can only exist in fixed orbits, or shells, and nowhere in between. Each shell has a fixed energy.
