Question

The atomic number of element X is 20. To which period and group of the modern periodic table, does this element X belong?

• A. Period 3, Group 2
• B. Period 3, Group 17
• C. Period 4, Group 16
• D. Period 4, Group 2

Answer 1

The correct option is D Period 4, Group 2

• The electronic configuration for atomic number 20 will be 2, 8, 8, 2.
• Hence the number of shells for a given element is 4 and the valence electron count is 2.
• The number of shells occupied by the electrons in the atom of an element decides its period number.​
• Hence, X belongs to the fourth period of the periodic table.
• Similarly, the number of valence electrons remains the same down the group. All elements in group 2 have 2 valence electrons.​
• So, X belongs to the second group of the periodic table.
• Therefore, element X belongs to Period 4, group 2.