- Period (periodic 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
quantum mechanicsexplains these periodic trends in properties in terms of electron shells. As atomic numberincreases, 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 electronsdetermine 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-blockof the periodic table, periodic trends across periods become significant, and the f-blockelements show a high degree of similarity across rows (particularly the lanthanides).
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