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

The elctronic configuration of calcium having atomic number 20 is 2, 8, 8, 2. Thus, calcium has 2 valence electrons in its outermost shell. Now, that element which has 2 valence electrons will have physical and chemical properties resembling that of calcium.

The electronic configuration of element having atomic number 12 is 2, 8, 2. It has 2 valence electrons just like calcium. So, the element having atomic number 12 will have physical and chemical properties resembling that of calcium.