What is the basis of the Modern Periodic Table? Why lithium with atomic number 3 and potassium with atomic number 19 placed in group one? What are the atomic numbers of the first two elements in the second group?

Explanation

a] Modern Periodic Table is based on the atomic number - the number of protons in an atom which decides its electronic configuration and hence its properties. All the elements are arranged in increasing order of their atomic number. Elements of similar properties are placed in the same vertical column.
b] Electronic configuration of Lithium is (2, 1) and potassium is (2, 8, 8, 1). Since both have one valence electron, they share similar chemical properties and are hence placed in group I.
c] The first element in the second group comes after Lithium with atomic number 3. Hence, its atomic number is 4. The second element in the second group has 8 electrons more (since periodicity occurs at intervals of 2, 8, 8, etc.) and hence, its atomic number is 4+8 = 12.