4 This question is about atomic structure.
4-1 Atoms contain subatomic particles.
Table 2 shows properties of two subatomic particles. Complete Table 2.
[2 marks]
Table 2
ANSWER
Name of particle | Relative Mass | Relative Charge |
Neutron | 1 | No charge |
Proton | 1 | +1 |
4-2 An element X has two isotopes.
The isotopes have different mass numbers.
Define mass number.
[1 mark]
ANSWER
Mass number is the sum of proton and neutron number.
4-3 Why is the mass number different in the two isotopes?
[1 mark]
ANSWER
Isotopes are atoms of the same element that have the same number of protons (i.e., atomic number, "Z") but a different number of neutrons, meaning that their mass number, "A", varies.
4-4 The model of the atom changed as new evidence was discovered.
The plum pudding model suggested that the atom was a ball of positive charge with electrons embedded in it.
Evidence from the alpha particle scattering experiment led to a change in the model of the atom from the plum pudding model.
Explain how.
[4 marks]
ANSWER
Plum-Pudding Model was replaced following some remarkable work from Hans Geiger and Ernest Marsden working with Ernest Rutherford. They fired alpha particles (He2+ ions) at a very thin piece of gold foil. They expected the particles to pass straight through the foil but a tiny fraction were deflected or even bounced back. This did not fit in with the plum-pudding model. Rutherford worked out that the scattering of some of the alpha particles meant that there must be a tiny, positive nucleus at the center of each atom. This new model was known as the nuclear model.
