Home ยป An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the Periodic Table does it belong? Periodic Table MCQs An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the Periodic Table does it belong? April 14, 2022 20 Views Q. An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the Periodic Table does it belong? A. Group 1 B. Group 4 C. Group 2 D. Group 3 Answer: Group 1