Developing models of atoms
Learn more on the history of the atom in this podcast.
Listen to the full series on ±«Óătv Sounds.
Daltonâs 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. Daltonâs 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
Thomsonâs 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 Thomsonâs âplum pudding modelâ:
- 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 âplanetary modelâ:
- 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
Bohrâs model (1913)
Niels Bohr improved Rutherfordâs model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.