The atom is not a solid ball. It has smaller particles, and scientists found them one by one. In this course we meet the electron, the proton and the neutron, and we follow the models of Thomson, Rutherford and Bohr, each one built on the failure of the one before.
3 lessons, about 1 h 20 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabAn atom has three particles. The electron is negative and very light. The proton is positive and the neutron has no charge. Both of these are about 1 u and sit in the nucleus.
Thomson pictured the atom as a positive sphere with electrons in it. Rutherford fired α particles at gold foil and found that the positive charge and most of the mass are in a tiny nucleus, with empty space around it.
Niels Bohr kept the nucleus of Rutherford's model. He said that electrons can move only in certain fixed orbits, called shells or energy levels: K, L, M and N, with n equal to 1, 2, 3 and 4. An electron in a shell does not lose energy, so the atom does not collapse. It jumps to a higher shell when it takes in a fixed amount of energy, and it gives out the same amount as light when it falls back. The shells hold at most 2, 8, 18 and 32 electrons. The model explains why atoms are stable and why hot gases give light of fixed colours.
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