For each element, the highest principle quantum number is the valence level of the electronic structure.
=>For Main Gp Elements => #Valence Electrons = Group Number
=>For Transition Metals and Inner Transition Metals => Valence level is the highest principle quantum number of the most stable electron configuration of ground state configuration.
Valence electrons in #CH_4#
V = C + 4H = 1(4) + 4(1) = 8
#C:[He]2s^(2)2p2# => 4 valence electrons
#H:1s^1# => 1 valence electron each Hydrogen => 4 valence #e^-#
Valence electrons in #C_2O_4^(2-)#
V = 2C + 4O + 2#e^-# = 2(4) + 4(6) + 2 = 34
(Add 1 valence electron for each - charge in anions)
#C:[He]2s^(2)2p^2# => 4 valence electrons each C => 8 valence #e^-#
#O:[He]2s^"2"2p^(4)# => 6 valence electrons each O => 24 valence #e^-#
#+2e^_# => excess of 2 #e^-#accounts for the -2 oxidation state
Valence electrons in #NH_4^+#
V = 1N + 4H - 1#e^-# 1(5) + 4(1) - 1 = 8
(Subtract 1 valence electron for each + charge in cations)
#N:[He]2s^"2"2p^3# => 5 valence electrons
#H:1s^1# => 1 valence electron each Hydrogen => 4 valence #e^-#
#-1e^-# => deficiency of 1 #e^-# accounts for +1 oxidation state
Valence electrons in #S_2O_3^(2-)#
V = 2S + 3O + 2#e^-# = 2(6) + 3(6) + 2 = 32
#S:[Ne]3s^"2"3p^(4)# => 6 valence electrons each S
=> 2(6) = 12 valence electrons
#O:[He]2s^"2"2p^(4)# => 6 valence electrons each Oxy
=> 3(6) = 18 valence electrons
#+2e^_# => excess of 2 #e^-#accounts for the -2 oxidation state
Valence electrons in #Ag(NH_3)_2^+#
V = Ag + 2N + 6H - 1#e^-#= 1 + 2(5) + 6(1) - 1 = 16
#Ag:[Kr]4d^(10)5s^1# => 1 valence electron (1 #e^-# in 5s orbital after stabilizing #4d ."orbital")#.
#N:[He]2s^"2"2p^3# => 5 valence electrons each Nitrogen => 2(5) = 10 valence electrons
#H:1s^1# => 1 valence electron each Hydrogen => 6 valence #e^-#
#-1e^-# => deficiency of 1 #e^-# accounts for +1 oxidation state
