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How are the horizontal periods of the periodic table organized?
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periodic table
Modern version of the periodic table of the elements. To learn an element's name, atomic number, electron configuration, atomic weight, and more, select one from the table.
The horizontal periods of the periodic table are organized primarily by increasing atomic number, which represents the number of protons in an atom's nucleus.1
Beyond this numerical sequence, the periods are structured based on the electronic configuration of the atoms:
- Electronic Shells: Each period corresponds to the successive occupation of orbitals within a specific valence shell of the atom. As you move down the table to successive periods, the principal quantum number (n) of the valence shell increases.2
- Subshell Filling: The periods represent the filling of specific subshells (s, p, d, and f). For example, the first period completes the K shell, while subsequent periods involve the filling of s and p subshells, followed by the d and f subshells in longer periods.3
- Period Lengths: The periods vary in length depending on the number of electrons required to fill these subshells. Each of the first six periods terminates at a noble gas, which possesses a stable, closed-shell electron configuration.23