Rutherford’s α scattering experiment The Nuclear Atom In the 19th century, Thomson’s model of the atom was popular; it suggested a plum pudding structure, where the atom was a positively charged globule with negatively charged electrons sprinkled in it. Rutherford’s α scattering experiment A stream of alpha particles from a radioactive source fired at a thin gold foil, only a few hundred atoms thick. The alpha particle source is encased in metal with a small aperture allowing a fine beam of α particles to emerge – the beam is collimated. Air in the apparatus pumped out to leave a vacuum because α particles are absorbed by only a few cm of air. Alpha particles detected when then hit a scintillating fluorescent screen. Each α particle gave a tiny flash of light and these could be counted by experimenters.

Conclusions from α scattering experiment The vast majority of α particles were not deflected at all: the atom must be mostly empty space Some α particles deflected through large angles: there must be a very small nucleus with a positive charge with a large electric field near to its surface. Alpha particles repelled: alpha particles are positively charged so the nucleus must be positively charged in order to create an electrostatic force of repulsion. Atoms are neutral overall: electrons must be on the outside of the atom separating one atom from the next.

The Nuclear Model Protons and neutrons make up the nucleus of the atom. Electrons move around the nucleus in a cloud – some closer and some further away from the nucleus. Particle Position Mass (kg) Charge (C) Proton Nucleus 1.673 × 10–27 1.6x10-19 Neutron 1.675 × 10–27 Electron Outside of nucleus 9.11 × 10–31 -1.6x10-19 Nucleon Number (A): total number of protons and neutrons Proton Number (Z): number or protons in the nucleus of an atom Nuclide: A specific combination of protons and neutrons in a nucleus Isotope: nuclei of same element with same number of protons but a different number of neutrons. 6C 12

Nuclear Density Origins of the Nuclear Strong Force Nuclear density is significantly larger than atomic density. This suggests that: Most of an atom’s mass is in its nucleus Nucleus is small compared to the atom Atom must contain lots of empty space Origins of the Nuclear Strong Force There a 2 forces already present in the nucleus: 1. Electrostatic Force of repulsion: all protons have an equal positive charge. The protons will repel each other with a force that can be calculated by Coulomb’s Law: 2. Gravitational Force of attraction: Newton’s Law of gravitation says that 2 objects will attract each other with a force equal to

Nuclear Strong Force The electrostatic force of repulsion is far bigger than the gravitational attraction. If these were the only 2 forces in the nucleus, it would fly apart. There must therefore be another attractive force to hold the nucleus together. This is the nuclear strong force. The nuclear strong force binds nucleons together. It is an attractive force that is larger than the electrostatic force of repulsion. It has a short range: it can only hold nucleons together when they are separated by up to 10fm. The strong force is repulsive at very small separations to stop the nucleus collapsing to a point.