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 16, 2022 40 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 4