In the modern 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? 

Explanation

Method I: The electronic configurations of the elements are :
`Ca (20) to (2, 8, 8, 2), (12) to (2, 8, 2), (19) to (2, 8, 8, 1), (21) to (2.8, 8, 3), (38) to (2, 8, 18, 8, 2)`
  Since elements with atomic number 12 and 38 have 2 valence electrons like calcium, these elements have physical and chemical properties resembling calcium.
Method 2: Since it is given that elements with atomic numbers 12, 19, 21, and 38 surround calcium, it is obvious that elements with atomic number 19 and 21 lie to the left and right of calcium respectively while elements with atomic number 1 2 and 38 lie at the top and bottom of calcium. Hence, elements with atomic numbers 12 and 38 fall in the same group as that of calcium and have physical and chemical properties resembling calcium. Moreover, we know that in the Modern Periodic Table the properties are repeated after 2, 8, 18, and 32 elements. Hence, it is logical that elements 12 and 38 should resemble calcium.