Answer
(a) Zinc has 2 valence electrons.
(b) Iodine has 7 valence electrons.
(c) Silicon has 4 valence electrons.
(d) Iron has 8 valence electrons. (2 in 4s subshell and 6 in 3d subshell)
Work Step by Step
The valence electrons of an atom are the electrons in the outermost shell of the atom which can participate in bonding.
(a) electronic configuration of Zinc is
1$s^{2}$2$s^{2}$2$p^{6}$3$s^{2}$3$p^{6}$4$s^{2}$3$d^{10}$
And thus, the number of electrons in the outermost shell i.e. n=4 shell(valence shell) is 2.
(b) electronic configuration of Iodine is
1$s^{2}$2$s^{2}$2$p^{6}$3$s^{2}$3$p^{6}$4$s^{2}$3$d^{10}$4$p^{6}$5$s^{2}$4$d^{10}$5$p^{5}$
And thus, the number of electrons in the outermost shell i.e. n=5 shell (valence shell) is 7.
(c) electronic configuration of Silicon is
1$s^{2}$2$s^{2}$2$p^{6}$3$s^{2}$3$p^{2}$
And thus, the number of electrons in the outermost shell i.e. n=3 shell (valence shell) is 4.
(d) electronic configuration of Iron is
1$s^{2}$2$s^{2}$2$p^{6}$3$s^{2}$3$p^{6}$4$s^{2}$3$d^{6}$
And thus, the number of electrons in the outermost shell i.e. n=4 shell (valence shell) is 2. However, Iron is a transition metal and it can use electrons in the d subshell as valence electrons. For transition metals, all the electrons outside the Noble gas core are considered as valence electrons. Thus in Iron, the Noble gas core is that of Argon ( a Noble gas core refers to the electronic configuration of the Noble gas just preceding the element) and the electronic configuration of Iron can also be written as
[Ar]4$s^{2}$3$d^{6}$
And hence, the total number of valence electrons is 2+6=8
