Question

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

Answer 1

(a) Phosphorus (P):
Atomic number = 15
The electronic configuration of P is: $1s^{2}2s^{2}2p^{6}3s^{2}3p^3$
The orbital picture of P can be represented as:



From the orbital picture, phosphorus has three unpaired electrons.

(b) Silicon (Si):
Atomic number = 14
The electronic configuration of Si is: $1s^{2}2s^{2}2p^{6}3s^{2}3p^2$
The orbital picture of Si can be represented as:


From the orbital picture, silicon has two unpaired electrons.

(c) Chromium (Cr):
Atomic number = 24
The electronic configuration of Cr is:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^5$
The orbital picture of chromium is:



From the orbital picture, chromium has six unpaired electrons.

(d) Iron (Fe):
Atomic number = 26
The electronic configuration is:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^6$
The orbital picture of chromium is:


From the orbital picture, iron has four unpaired electrons.

(e) Krypton (Kr):
Atomic number = 36
The electronic configuration is:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^6$ The orbital picture of krypton is:


Since all orbitals are fully occupied, there are no unpaired electrons in krypton.