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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 number 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 

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods (particularly the lanthanides).

v • d • e Periodic tables Layouts Standard · Inline f-block · Vertical · Full names · Names and atomic weights · Text for last · Large table · Metals and nonmetals · Blocks · Valences · Extension beyond the 7th period · Electron configurations · Atomic weights · Electronegativities · Alternatives · Crystal structure Lists of elements by Name · Atomic symbol · Atomic number · Atomic weight · Name etymology (after places, after people) · Discovery Boiling point · Melting point · Density · Oxidation state · Abundance (in humans) · Nuclear stability · Hardness Groups 1 (Alkali metals) · 2 (Alkaline earth metals) · 3 · 4 · 5 · 6 · 7 · 8 · 9 · 10 · 11 · 12 · 13 (Boron group) · 14 (Carbon group) · 15 (Pnictogens) · 16 (Chalcogens) · 17 (Halogens) · 18 (Noble gases) Periods 1 · 2 · 3 · 4 · 5 · 6 · 7 · 8 Other element categories Metals · Transition metals (1st row) · Metalloids · Nonmetals · Lanthanoids · Actinoids · Rare earth elements · Platinum group metals (PGMs) · Post-transition metals Blocks s-block · p-block · d-block · f-block

The periodic table of elements is organized by periods and families or groups this means that the periods stand for the valence electrons or negatively charged atoms known as electrons in the electrons shells in the electron cloud.