Period: definition in chemistry
Period: A horizontal row of the periodic table. There are seven periods, and the elements in each are listed in order of increasing atomic number.
The period number equals the number of the outermost occupied electron shell in the atoms of that row: sodium, in period 3, has electrons in three shells. Moving across a period, protons are added to the nucleus while electrons fill the same shell, so atoms generally get smaller and hold their electrons more tightly. As a result, elements change from reactive metals on the left to nonmetals and finally a noble gas on the right.
Examples
- Period 1 — only hydrogen and helium
- Period 2 — eight elements, from lithium to neon
- Period 4 — 18 elements, from potassium to krypton
- Periods 6 and 7 — 32 elements each, including the lanthanides and actinides
On the periodic table
Period numbers run from 1 to 7 down the left edge of the table, and the two rows printed separately at the bottom belong to periods 6 and 7.