History of the Atom Flashcards
Who described atoms as solid spheres?
John Dalton
What did Dalton say the different spheres were made up of?
Elements
Who first said atoms weren’t solid spheres?
When?
- J J Thomson
- 1897
What did JJ Thomson’s measurements of charge and mass show?
An atom must contain even smaller, negatively charge electrons
What was Thomson’s theory known as?
The plum pudding model
What does the plum pudding model show?
The atom as a ball of positive charge with randomly placed electrons inside
What experiment did Ernest Rutherford and his student conduct?
When?
- Alpha particles scattering
- 1909
How did Rutherford conduct his experiment of alpha particles?
Firing positively charged alpha particles at an extremely thin sheet of gold
What would the plum pudding model have proved about Rutherford’s experiment?
That the particles should have passed straight through the sheet or slightly deflected at most
Why was the plum pudding model proved wrong?
A small number of particles were deflected backwards
What was Rutherford’s new model called?
The nuclear model of the atom
What does the nuclear model of the atom show?
- There’s a small, positively charged nucleus at the centre of the atom, where most of the mass is concentrated
- Negatives electrons surround the nucleus in a ‘cloud’, creating empty space
What does the nuclear model of the atom say about what happens when alpha particles were fired at the sheet of gold?
- When the particles came near the positive charge of the nucleus, they were slightly deflected
- When the particles came straight at the nucleus, they were deflected backwards
- Particles that were fired far from the nucleus were shot straight through empty space
What did Niels Bohr suggest about the nuclear model of the atom?
If the electrons were in a ‘cloud’, the atom would collapse. He said the electrons must be contained in cells.
What does Bohr propose about the state of electrons?
The electrons orbit the nucleus in fixed shells, a fixed distance from the nucleus
