In the periodic table of the elements, a period is a horizontal row of the table.
The elements are laid out in a series of rows so that those with similar properties line up in vertical columns: this reflects the periodic recurrence of similar properties as the atomic weight increases.
Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, electron shells are filled in roughly this order. The filling of each shell corresponds to a row in the table.
1s
2s 2p
3s 3p
4s 3d 4p
5s 4d 5p
6s 4f 5d 6p
7s 5f 6d 7p
6f 7d 7f
...
Hence the structure of the periodic table. Since the valence 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. In the d-block of the periodic table, periodic trends across periods become significant, and the f-block elements show a high degree of similarity across rows (particularly the lanthanides).
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