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| Number of electrons | 30 | 25 | 20 | 15 | 10 | 5 |
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| Number in successive rings | 15 | 13 | 12 | 10 | 8 | 5 |
| | 10 | 9 | 7 | 5 | 2 | |
| | 5 | 3 | 1 | | | |
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In the next table is given the possible series of arrangements of electrons which can have an outer ring of 20:—
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| Number of electrons | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 |
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| Number in successive rings | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 |
| | 16 | 16 | 16 | 17 | 17 | 17 | 17 | 17 | 17 |
| | 13 | 13 | 13 | 13 | 13 | 13 | 14 | 14 | 15 |
| | 8 | 8 | 9 | 9 | 10 | 10 | 10 | 10 | 10 |
| | 2 | 3 | 3 | 3 | 3 | 4 | 4 | 5 | 5 |
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The smallest number of electrons which can have an outer ring of 20 is 59, while 67 is the greatest.
The various arrangements of electrons can be classified into families, in which the groupings of the electrons have certain features in common. Thus the group of 60 electrons consists of the same arrangement of electrons as the group of 40 with the addition of an outer ring of 20 electrons; the group of 40 is the same as the group of 24 with an additional ring outside; and the group of 24 in turn is the same as the group of 11 with an extra ring. A series of model atoms may be formed in this way, in which each atom is derived from the preceding member by an additional ring of electrons. Such atoms would be expected to possess many properties in common, and would correspond to the elements in the same vertical column of the periodic table of Mendeléef.
Different arrangements of electrons vary widely in stability. Some may acquire an extra electron or two and yet remain stable, others readily lose an electron without disturbing their stability. The former would correspond to an electro-negative atom, the latter to an electro-positive.
Certain arrangements of electrons are stable if the electrons move with an angular velocity greater than a certain value, but
