7.2 The discovery of the nucleus

Question 1

What arrangement did Geiger and Marsden use for the gold foil experiment

Answer 1

• apparatus in vacuum chamber: prevents air molecules absorbing alpha particles
• detector: microscope focused on small glass plate
• when alpha particles hit the plate, a spot of light was observed
• detector moved to different positions

Question 2

What did the results of the gold foil experiment show

Answer 2

• most alpha particles passed straight through the metal foil
• number of particles deflected per minute decreased as the angle of deflection increased
• 1 in 1000 particles were deflected by more than 90 degrees

Question 3

What did Rutherford already know about atoms

Answer 3

• that alpha particles are positively charged
• that these radius of an atom is 10^-10 m

Question 4

What did Rutherford deduce from the results of the gold foil experiment

Answer 4

That there was a nucleus at the centre of every atom that is:
- much smaller than the atom (because most alpha particles passed without deflection)
- where most of the mass of the atom is located

Question 5

Why was Rutherford’s model of the atom widely accepted?

Answer 5

• it agreed exactly with the measurements Geiger and Marsden made in their experiments
• it explained radioactivity in terms of changed happening to an unstable nucleus when it emits radiation
• it predicted the existence of the neutron, which was later discovered

Question 6

What was the plum pudding model

Answer 6

Atoms had positively charged matter spread out evenly with the electrons buried inside

Question 7

What theory did Niels Bohr put forward

Answer 7

That electrons orbit the nucleus at specific distances and energy levels

Question 8

What did Bohr’s model of the atom show

Answer 8

electrons in an orbit can move to another orbit by:
- absorbing electromagnetic radiation to move away from the nucleus
- emitting electromagnetic radiation to move closer to the nucleus
his calculations based on his atomic model agreed with experimental observations of light emitted by atoms

Question 9

What did more alpha scattering experiments show

Answer 9

• the hydrogen nucleus has the least amount of charge
• the charge of any nucleus is shared equally between a whole number of smaller particles, each with the same amount of positive charge

Question 10

What was the name proton given to?

Answer 10

The hydrogen nucleus

Question 11

How did scientists realise that neutrons existed?

Answer 11

• the mass of every nucleus (except for hydrogen) is bigger than the total mass of its protons
• so there must be an uncharged particle with about the same mass as a proton in every nucleus (except for hydrogen)

Question 12

Why did scientists know the proton-neutron model of the atom was correct

Answer 12

It explains all the mass and charge values of every nucleus

Question 13

Who found direct evidence of the neutron and when

Answer 13

James Chadwick, 20 years after Rutherford’s discovery of the nucleus