Image: Thomson atom electron arrangements

Description: A diagram presented by J. J. Thomson to show how he believed electrons are arranged in atoms. (Royal Institution of Great Britain, March 1905)Excerpt:Let us now consider the kind of atom we could build up out of corpuscles and positive electricity. The mathematical investigation of this problem leads to the following results. The simplest atom containing 1 corpuscle would have 1 corpuscle at the centre of the sphere of positive electrification; the 2 corpuscle atom would have the 2 corpuscles separated by a distance equal to the radius of this sphere; the 8 corpuscle atom would have the 3 corpuscles at the points of an equilateral triangle, whose side is equal to the radius of the sphere; 4 corpuscles would be at the corners of a regular tetrahedron, whose side is equal to the radius of the sphere; 5 corpuscles are situated, 4 at the corners and 1 at the centre of a tetrahedron; 6 at the corners of an octahedron; 7 and 8 are more complicated, as the simplest arrangements for 7 and 8, an octahedron with 1 at the centre and a cube, are both unstable; and for 7 we have a ring of 5 in one plane with 2 on a line through the centre at right angles to the plane; and 8 we have the octahedron with 2 inside.
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