5.3.1 The Nuclear Atom

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

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cs Alpha Particle Scattering

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cs Gold Foil Experiment

What were Geiger and Marsden’s results?

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.

2. Some alpha particles were slightly deflected by the gold foil.

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cs 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.

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cs 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?

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cs Gold Foil Experiment

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cs Videohttp://www.youtube.com/watch?v=u0HMYLSUzTU

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

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cs Structure and size of the atomUsing Page 181 in Physics 2:

describe the basic atomic structure of the atom

and the relative sizes of the atom and the nucleus

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cs Structure and size of the atomAtom: 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 proton ~ 10-15m radius of nucleus ~ 10-15m to 10-14m radius of atom ~ 10-10m size of molecule ~ 10-10m to 10-6m

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cs 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

… or …if the nucleus was the size of a marble, the orbiting

electron would be a grain of sand 800m away… or … if the nucleus was a shopping trolley in Trafalgar

Square, the electrons would be orbiting around the M25

(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)

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cs 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

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cs 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

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cs Forces in the nucleus

Two protons in the nucleus of an atom are separated by 1.6 x 10-15 m.

1. Calculate the force of electrostatic repulsion between them, and the force of gravitational attraction between them.

2. Is the force of gravity enough to balance the electric repulsion tending to separate them?

3. What does this suggest to you about the forces between protons in the nucleus?

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Newton’s law of gravitation:

where M and m = mass of each objectr = distance between each objectG = gravitational constant

Forces in the nucleus

Coulomb’s law:

where Q and q = point chargesr = distance between point charges ε0 = permittivity of free space

F = GMm r2

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cs Forces in the nucleus

Data:

Proton Q = 1.610-19C m = 1.6710-27kgε0 = 8.8510-12 Fm-1

G = 6.67 x 10-11 Nm2kg-2

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

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cs 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

(e) estimate the density of nuclear matter

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cs Density of Nuclear Matter

Mass of proton mp = 1.67 x 10-27 kgRadius of proton r = 0.80 x 10-15 m

Volume of proton = 4 πr3

3

Density (kg m3) = mass (kg) volume (m3)

(f) define proton and nucleon number

(g) state and use the notation for the representation of nuclides

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cs Nucleons and electrons

Proton number:The number of protons (which is the same as number of electrons in a neutral atom).

Nucleon number:The number of protons plus the number of neutrons in a neutral atom. Element

symbol

A

X Z

(h) define and use the term isotopes

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cs 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:

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cs 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.

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cs Isotopes

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

of protons.

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cs Assessment

Chapter 12 SAQ’s 1 to 11

End of Chapter 12 questions 1 - 3

Atomic Structure worksheet questions 1 – 5 and 8 – 10

(i) use nuclear decay equations to representsimple nuclear reactions

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