Periods in chemistry refer to the horizontal rows of elements in the periodic table, which represent all 118 elements across 7 rows.
The periods run from left to right, with each consecutive element increasing in atomic number by 1.
Periods 1, 2, and 3 have 2, 8, and 8 elements, whereas periods 4, 5, 6, and 7 have 18, 18, 32, and 32 elements.

All elements in a period have the same number of electron shells in their atoms. Though the elements may share the same period, they may not share the same chemical properties, as evidently each row starts (except 1) with the highly reactive alkali metals and ends with an unreactive noble gas, and therefore, vary immensely in their reactivities.
