Period (periodic table)
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In the periodic table of the elements, a period is a horizontal row of the table. A group, or family, is a vertical column of the table.
The elements are arranged in a series of rows so that those with similar properties appear in vertical columns. This arrangement reflects the periodic recurrence of similar properties as the atomic weight increases. For example, the alkaline metals lie in one group (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at the noble gas electronic configuration.
Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown below. The filling of each shell corresponds to a row in the table.
1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p
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Elements in periods (rows) and groups (columns) share properties, but in two different ways. Elements in the same period have the same number of energy levels. Elements in a group have the same number of electrons in the outermost energy level (the valence electrons), leading to similar chemical properties.
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, 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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