Scattering to determine structure 58-59

Question 1

Explain Rutherford’s experiment. What did they find from the results?

Answer 1

A stream of alpha particles from a radioactive source was fired at very thin gold foil. It was found that alpha particles occasionally scattered at angles greater than 90*. This can only be possible if they’re striking something more massive than themselves.

Question 2

What four important conclusions did Rutherford get from his experiment?

Answer 2

1) Most of the fast, charged alpha particles went straight through the gold foil. Therefore the atom is mostly empty space. 2) Some of the alpha particles are deflected back through significant angles, so the centre of the atom must be tiny but contain a lot of mass. Rutherford named this the nucleus. 3) The alpha particles were repelled, so the nucleus most have a positive charge. 4) Atoms are neutral overall so the electrons must be on the outside of the atom - separating one atom from the next.

Question 3

What are nucleons?

Answer 3

Protons and neutrons.

Question 4

Explain how a rough estimate of the size of an atom can be found through firing an alpha particle at an atom and knowing its initial kinetic energy.

Answer 4

An alpha particle that ‘bounces back’ and is deflected through 180* will have stopped a short distance from the nucleus . It does this at the point where its electrical potential energy equals its initial kinetic energy.

Question 5

What is the equation that allows us to calculate how close an alpha particle can get to a nucleus knowing its kinetic energy.

Answer 5

Initial K.E = E_elec = Q_atomq_alpha/4πE₀r

• To find the charge on the nucleus, Q_atom, you need to know the atom’s proton number, Z. A proton has a charge of +e so the charge of the nucleus must be +Ze.