1.1- Developing ideas of the atom

Question 1

Rutherford’s gold foil experiment

Answer 1

• Involved firing alpha particles (which are positively charged) at a sheet of thin gold foil inside a vacuum.
• According to the plum pudding model, the positive ‘pudding’ would slightly deflect most of the particles.
• Contrary to expectations, most alpha particles passed straight through, with a few deflecting at large angles or even bouncing back.

Question 2

What are models?

Answer 2

• a simplified depiction of reality to help us understand a concept

Question 3

Daltons solid sphere model (proposed by who and what did he propose)

Answer 3

• Proposed by John Dalton, this model suggested that atoms were indivisible, solid spheres.
• It also suggested that different elements were made up of different types of these spheres.

Question 4

Bohr’s Model (introduction of electron shells)

Answer 4

• Electrons orbit the nucleus in specific energy levels, or shells.
• They can jump between these shells by absorbing or emitting light of particular wavelengths.

Question 5

Modifications to the nuclear model (discovery of the proton and neutron)

Answer 5

• Research by Henry Moseley showed that the nuclear charge increases by one unit from one element to the next.
• This insight led Rutherford to identify positively charged particles within the nucleus, which he namedprotons.
• The differences in nuclear charges among elements could now be explained by the presence of varying numbers of protons.

Question 6

Thomson’s plum pudding model (what did he discover and prove, what did his model show)

Answer 6

• The discovery of the electron by J.J. Thomson proved that atoms were divisible.
• His ‘plum pudding’ model depicted atoms as spheres of positive charge with negative electrons randomly embedded throughout, similar to the raisins in a plum pudding.

Question 7

Discovery of the neutron

Answer 7

• James Chadwick later discovered theneutron, which provided the mass needed to account for the discrepancy in the nucleus’s weight.

Question 8

Conclusions of the gold foil experiment

Answer 8

• The fact that most alpha particles passed through the gold foil undeflected indicated that atoms are mostly empty space.
• The small number of particles that deflected at large angles or bounced back suggested the presence of a tiny, dense, positively charged nucleus at the centre of the atom. This nucleus was responsible for the strong deflection of the positively charged alpha particles due to electrostatic repulsion.
• Since the atom was mostly empty space with a dense nucleus, Rutherford proposed that negative electrons must orbit the nucleus in a ‘cloud’ to maintain the atom’s overall neutrality.