Indicate the number of unpaired electrons in:
$(a)$ $P$ $(b)$ $Si$ $(c)$ $Cr$ $(d)$ $Fe$ and $(e)$ $Kr$

(N/A) Phosphorus $(P):$
Atomic number $= 15$
Electronic configuration: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{3}$
Number of unpaired electrons $= 3$
$(b)$ Silicon $(Si):$
Atomic number $= 14$
Electronic configuration: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{2}$
Number of unpaired electrons $= 2$
$(c)$ Chromium $(Cr):$
Atomic number $= 24$
Electronic configuration: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{5}$
Number of unpaired electrons $= 6$
$(d)$ Iron $(Fe):$
Atomic number $= 26$
Electronic configuration: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{6}$
Number of unpaired electrons $= 4$
$(e)$ Krypton $(Kr):$
Atomic number $= 36$
Electronic configuration: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{6}$
Number of unpaired electrons $= 0$