Account for the following :
(a) Elements C, N, O and F are placed in the second period of the periodic table.
(b) Elements of group 17 are monovalent.

Explanation

(a) All these elements have two electron shells. Therefore, these are placed in the second period.
C (Z=6) 2, 4 , N (Z = 7) 2, 5 , O (Z = 8) 2, 6 , F (Z = 9) 2, 7
(b) All the elements included in the group 17 have 7 valence electrons in their atoms. These atoms need only one electron to acquire the electronic configuration of nearest noble gas atom. Therefore, these are monovalent.