Elements present in the same period have the same number of shells which is equal to the period number. A period is a row of the periodic table that is horizontal. The periodic table contains seven periods, each of which starts at the left.
On moving from left to right in a given period, the number of electrons in the valence shell increases from one to eight while the number of shells remains the same.
Number of Elements in a Period
The first period contains only two elements 1Hand2He and is known as the shortest period.
The second period (3Li to 10Ne) and the third period (11Na to 18Ar) contain 8 elements each and are known as short periods.
The fourth period (19K to 36Kr) and the fifth period (37Rb to 54Xe) contain 18 elements each and are called long periods.
The sixth period contains 32 elements (55Cs and 86Rn) and is also known as the longest period.
The seventh period is an incomplete period.
(After the recent discoveries of the new elements and their addition to the periodic table, the seventh period is officially complete)