Atomic structure

Question 1

Describe the plum pudding model (currant bun)

Answer 1

It is a sphere of positive charge with randomnly embedded negative electrons.

Question 2

Describe the nuclear model

Answer 2

• It has a small centrally situated positive nucleus.
• Electrons were found in the energy levels (“shells”)
• It also contains neutrons discovered by James Chadwick
• Electrons occupy energy levels (2,8,8,2)

Question 3

Describe the results and conclusions of the alpha scattering experiment.

Answer 3

• Most of the alpha particles passed straight through the gold foil.
  conclusion: The atom has space.
• A few particles approached the gold foil and deflected/ deviated at right angles.
  conclusion: The atom has charge (alpha is positive and the nucleus of an atom is positive- repulsion)
• One or two of the alpha particles were reflected when striking the gold foil.
  conclusion: The mass of the atom is concentrated in the nucleus- change in momentum.

Question 4

How did the atomic structure change?

Answer 4

• JJ Thomson first proposed that the atom consisted of a ‘pudding’ with a positive charge and negative electrons, ‘plums’ embedded throughout.
• Geiger and Marsden did the alpha scattering experiment which lead to the nuclear model made by Rutherford.
• Niels Bohr discovered that electrons orbitted the nucleus at specific distances- Bohr model.
• James Chadwick discovered the neutron.

Question 5

Charges in an atom

Answer 5

Proton- +1
Neutron- 0
Electron- -1

Question 6

Relative masses in an atom

Answer 6

Proton- 1
Neutron- 1
Electron- 1/2000 or undefined

Question 7

Q: Why is an atom neutral?

Answer 7

The number of electrons is equal to the number of protons. The charge on each proton is balanced by the charge on each electron and the net charge is zero. The net charge is zero and neutrons have zero charge.

Question 8

What is the difference between the plum pudding model of the atom and the nuclear model of the atom?

Answer 8

• The plum pudding model has positive charge distributed throughout the atom whereas the nuclear model has a positively charged nucleus only in the centre of the atom.
• The plum pudding model has electrons embedded throughout the atom whereas the nuclear model has electrons orbitting the nucleus in energy levels/ shells.

Question 9

How big are atoms?

Answer 9

They are very small, having a radius of 0.1 nm

Question 10

How big is the radius of a nucleus?

Answer 10

The radius of a nucleus is less than 1/10000 the radius of an atom (around 1x 10^-14m)

Question 11

What are isotopes?

Answer 11

Atoms of the same element which have the same atomic number (protons) but different mass number (neutron number)

Question 12

What is the relative atomic mass?

Answer 12

The mass of one atom of an element relative to the carbon-12 isotope