IB Physics Sub-topic E1 Notes

Evidence for atomic structure

Topic E focuses on nuclear physics. To fully understand this, you need to know the model of atomic structure. In early atomic modelling, three individuals were very important:

1. Democritus considered the structure of matter, concluding that if one kept dividing matter, one would reach a particle that is indivisible, the atom.
2. Dalton then took this one step further and decided that matter could be divided into different elements, which themselves were composed of atoms of different masses.
3. Thomson fired electron beams at electromagnetic fields, determining the size and charge of an electron in the process. Using this knowledge, he developed the plum pudding model of the atom, which was composed of electrons in a matrix of positive matter.

Following these early models, Rutherford uncovered the nucleus of an atom via the Rutherford-Geiger-Marsden experiment.

1. In this, Helium nuclei were fired at a thin gold film to attempt to find structures.
2. Some particles went straight through the film, but others were scattered at huge angles.
3. The nuclei were already known to be positively charged, so the scattering suggested the presence of a strong positive charge in the gold film, the nucleus.
4. However, the straight paths suggested there was empty space between the nuclei.
5. This formed the basis of the Rutherford model: atoms consist of a positive nucleus surrounded by electrons in free space.

The Bohr model

Finally, Niels Bohr discovered electron energy levels, using this to complete our basic model of the atom, known as the Bohr model. The IB requires you to know the Bohr model of the atom, which is composed of:

1. A dense positively charged nucleus with protons and neutrons, termed nucleons. 
2. Negatively charged electrons in orbitals around the nucleus, arranged into energy levels/shells.

Each particle has a mass and charge, shown in the table below: