question_answer 22)

In the modern periodic table, calcium (atomic number 20) is surrounded by elements with atomic number 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium? 

Answer:

Only those elements are placed in the same group in which the gaps of atomic numbers are: 8,8,18,18,32. If we look at the atomic numbers of the elements that are listed, it becomes clear that the elements with atomic numbers 12, 20 (Ca), 38 fit into this pattern. They are  placed in the same group and have also similar physical and chemical properties.