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In the periodic table of the elements,
a period is a row of the table.
The number of electron shells an atom has determines what period it
belongs to. Each shell is divided into different subshells, which as atomic
number increases are filled in roughly this order:
1s
2s 2p
3s 3p
4s 3d 4p
5s 4d 5p
6s 4f 5d 6p
7s 5f 6d 7p
8s 5g 6f 7d 8p
...
Hence the structure of the periodic table. Since the outermost electrons determine chemical properties, those tend to be
similar within periodic table groups.
Elements adjacent to one another within a group have similar physical properties, despite their significant differences in
mass. Elements adjacent to one another within a period have similar mass but different properties.
See also:
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