1. 5.3.1 The Nuclear Atom

2. (a) describe qualitatively the alpha-particle scattering experiment and the evidence this provides for the existence, charge and small size of the nucleus (HSW)

3. Alpha Particle Scattering

4. Gold Foil Experiment What were Geiger and Marsden’s results?
2. Some alpha particles were slightly deflected by the gold foil.
3. A few alpha particles were bounced back from the gold foil.
1. Most alpha particles
went straight through
the gold foil, without
any deflection.

5. How did Rutherford interpret the results?
Rutherford had expected all the alpha radiation to pass through the gold foil. He was surprised that some alpha particles were deflected slightly or bounced back.
The ‘plum pudding’ model could not explain these results, so Rutherford proposed his ‘nuclear’ model of the atom.
He suggested that an atom is mostly empty space with its positive charge and most of its mass in a tiny central nucleus.
Electrons orbited this nucleus at a distance, like planets around the Sun.

6. How can these results be explained in terms of atoms?
Gold Foil Experiment
The experiment was carried out in a vacuum, so deflection of the alpha particles must have been due to the gold foil.
How can these results be explained in terms of atoms?

7. Gold Foil Experiment

8. Video

9. (b) describe the basic atomic structure of the atom and the relative sizes of the atom and the nucleus

10. Structure and size of the atom
Using Page 181 in Physics 2:
describe the basic atomic structure of the atom
and the relative sizes of the atom and the nucleus

11. Structure and size of the atom
protons and neutrons make up the nucleus of the atom
the electrons move around the nucleus in a cloud, some closer to and some further from the centre of the nucleus
Scale:
radius of proton ~ 10-15m
radius of nucleus ~ 10-15m to 10-14m
radius of atom ~ 10-10m
size of molecule ~ 10-10m to 10-6m

12. Structure and size of the atom
… or …
if the nucleus was a grape seed in the centre circle, the electrons would be orbiting around the outside of Wembley Stadium
if the nucleus was the size of a marble, the orbiting electron would be a grain of sand 800m away
if the nucleus was a shopping trolley in Trafalgar Square, the electrons would be orbiting around the M25

13. (c) select and use Coulomb’s law to determine the force of repulsion, and Newton’s law of gravitation to determine the force of attraction, between two protons at nuclear separations and hence the need for a short range, attractive force between nucleons (HSW) 13

14. Forces in the nucleus Nucleus consists of protons (+e) and neutrons
Logic dictates that the +e protons should repel each other
The fact that they don’t indicates another force in the nucleus that holds the nucleons together
This force is called the strong nuclear force
It only acts over small (10-14m) distances

15. Forces in the nucleus There are two other forces in the nucleus:
electrostatic
gravitational
The strength of each force can be calculated by:
repulsive electrostatic – Coulomb’s law
attractive gravitational – Newton’s laws

16. Forces in the nucleus
Two protons in the nucleus of an atom are separated by 1.6 x m.
Calculate the force of electrostatic repulsion between them, and the force of gravitational attraction between them.
Is the force of gravity enough to balance the electric repulsion tending to separate them?
What does this suggest to you about the forces between protons in the nucleus?

17. Forces in the nucleus Coulomb’s law: where Q and q = point charges
r = distance between point charges
ε0 = permittivity of free space
Newton’s law of gravitation: where M and m = mass of each object r = distance between each object G = gravitational constant
F = GMm r2

18. Forces in the nucleus
Data: Proton Q = 1.610-19C m = 1.6710-27kg ε0 = 8.8510-12 Fm-1 G = 6.67 x Nm2kg-2

19. (d) describe how the strong nuclear force between nucleons is attractive and very short-ranged19

20. Strong Nuclear Force Acts over very short distances (10-14m)
Attractive force
Only reaches adjacent nucleons
Large nucleus not held together as tightly as a small nucleus

21. (e) estimate the density of nuclear matter21

22. Density of Nuclear Matter
Mass of proton mp = 1.67 x kg Radius of proton r = 0.80 x m Volume of proton = 4 πr3 3 Density (kg m3) = mass (kg) volume (m3)

23. (f) define proton and nucleon number (g) state and use the notation for the representation of nuclides 23

24. Nucleons and electrons
Nucleon number:
The number of protons plus the number of neutrons in a neutral atom. A X Z
Element symbol
Proton number:
The number of protons (which is the same as number of electrons in a neutral atom).

25. (h) define and use the term isotopes25

26. Isotopes
All carbon atoms have the same number of protons, but not all carbon atoms are identical. Although atoms of the same element always have the same number of protons, they can have different numbers of neutrons. Atoms that differ in this way are called isotopes. For example, carbon exists as three different isotopes: carbon-12, carbon-13 and carbon-14:

27. Isotopes
For example, carbon exists as three different isotopes: carbon-12, carbon-13 and carbon-14:
Nucleon number is different
Proton number is the same
Potassium is another element that exists as three different isotopes: potassium-39, potassium-40 and potassium-41.

28. Isotopes
Isotopes are nuclei of the same element with a different number of neutrons but the same number of protons.

29. Assessment Chapter 12 SAQ’s 1 to 11 End of Chapter 12 questions 1 - 3
Atomic Structure worksheet questions 1 – 5 and 8 – 10

30. (i) use nuclear decay equations to represent simple nuclear reactions30

31. (j) state the quantities conserved in a nuclear decay.31