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)

Alpha Particle Scattering

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.

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.

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?

Gold Foil Experiment

Video http://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

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

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

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

(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

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

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

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

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

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 10-11 Nm2kg-2

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

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 21

Density of Nuclear Matter Mass of proton mp = 1.67 x 10-27 kg Radius 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 23

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).

(h) define and use the term isotopes 25

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:

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.

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

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 represent simple nuclear reactions 30

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