Explanation

• The 4th and 5th periods in the periodic table are known as the long periods.
• These periods contain 18 elements each, making them the longest periods in the periodic table.

Important Points:

• Short periods: 1st and 2nd periods (2 and 8 elements, respectively)
• Long periods: 4th and 5th periods (18 elements each)
• Normal periods: 3rd and 6th periods (8 and 12 elements, respectively)