In the periodic table of the elements, a period is a horizontal row of the table.

The number of electron shells an atom has determines what period it belongs to. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order:

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 8s 5g 6f 7d 8p ...

Hence the structure of the periodic table. Since the outermost 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. Elements adjacent to one another within a period have similar mass but different properties.

See also:

• Period 1 element
• Period 2 element
• Period 3 element
• Period 4 element
• Period 5 element
• Period 6 element
• Period 7 element
• Period 8 element
• Period 9 element

Periodic table

Periode (Periodieketabel) | دورة جدول دوري | Periodu de la tabla periódica | Период на периодичната система | Període de la taula periòdica | Periode des Periodensystems | Periodo de la tabla periódica | Periodo de la perioda tabelo | Taula periodikoaren periodo | Période du tableau périodique | 주기율표 주기 | Periode tabel periodik | Lota (lotukerfið) | Periodo della tavola periodica | Periódusos rendszerbeli periódus | Periode (scheikunde) | 元素の周期 | Periodane i periodesystemet | Período (química) | Perioadele tabelului periodic | Периода периодног система елемената | Periode tabel periodik | Jakso (jaksollinen järjestelmä) | Periodiska systemets perioder | คาบในตารางธาตุ | 元素周期