Developing models of atoms
Learn more on the history of the atom in this podcast.
Listen to the full series on 大象传媒 Sounds.
Dalton鈥檚 model (1803)
John Dalton thought that all matter was made of tiny A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. called The smallest part of an element that can exist., which he imagined as tiny solid balls. Dalton鈥檚 model included these ideas:
- atoms cannot be broken down into anything simpler
- the atoms of a given A substance made of one type of atom only. are identical to each other
- the atoms of different elements are different from one another
- during chemical reactions atoms rearrange to make different substances
Thomson鈥檚 model (1897)
J.J. Thomson discovered the Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. Atoms are neutral overall, so in Thomson鈥檚 鈥榩lum pudding model鈥:
- atoms are spheres of positive charge
- electrons are dotted around inside
The Geiger-Marsden experiment (1909 - 1911)
Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively-charged particles at very thin gold foil. These particles should have passed straight through, according to the plum pudding model. However, many of them changed direction. Ernest Rutherford explained these results in his 鈥榩lanetary model鈥:
- atoms have a central, positively charged The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. with most of the mass
- electrons orbit the nucleus, like planets around a star
Bohr鈥檚 model (1913)
Niels Bohr improved Rutherford鈥檚 model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.