Chemistry Reference
In-Depth Information
Boron
Carbon
Nitrogen
Oxygen
2
p
2
p
2
p
2
p
2
s
2
s
2
s
2
s
1
s
1
s
1
s
1
s
Figure 2.4
Electronic configurations: energy diagrams
2.4 Electronic configurations
obligatory, e.g. O: 1
s
2
2
s
2
2
p
x
2
2
p
y
1
2
p
z
1
. There is no
hidden meaning in allocating electrons to 2
p
x
first.
In any case, we are unable to identify which of these
orbitals is filled first.
Alternatively, we can use the even more infor-
mative energy diagram (Figure 2.4). Electrons with
different spin states are then designated by upward
(
↑
)
or downward
(
↓
)
pointing arrows. Note particu-
larly that the
noble gas
neon has enough electrons to
fill the orbitals of the '2' shell completely; it has a
total of eight electrons in these orbitals, i.e. an
octet
.
In the case of helium, the '1' shell orbital is filled
with two electrons. The next most stable electronic
configurations are those of argon, 1
s
2
2
s
2
2
p
6
3
s
2
3
p
6
,
and then krypton, 1
s
2
2
s
2
2
p
6
3
s
2
3
p
6
4
s
2
3
d
10
4
p
6
.The
filled electron shells are especially favourable and
responsible for the lack of reactivity of these two
elements. Attaining
filled shells
is also the driving
force behind bonding.
Each atomic orbital can accommodate just two
electrons, provided these can be paired by virtue of
having opposite spin quantum numbers. If the spins
are the same, then the electrons must be located in
different orbitals. We can now describe the
electronic
configuration
for atoms of interest in the first two
rows of the periodic table. Electrons are allocated
to atomic orbitals, one at a time, so that orbitals
of one energy level are filled before proceeding to
the next higher level. Where electrons are placed
in orbitals of the same energy (degenerate orbitals,
e.g.
p
orbitals) they are located singly in separate
orbitals before two electrons are paired. Further, the
electronic configuration with the greatest number of
parallel spins (same spin quantum number) results in
the lowest energy overall.
The electronic configuration can be expressed as a
list of those orbitals containing electrons, as shown
below. Although it is usual just to indicate the number
of electrons in
p
orbitals, e.g. C: 1
s
2
2
s
2
2
p
2
as in
the first column, it is more informative to use the
second column designation, i.e. C: 1
s
2
2
s
2
2
p
x
1
2
p
y
1
,
where the non-pairing of electrons is emphasized.
As more 2
p
electrons are allocated, pairing becomes
2.5 Ionic bonding
The simplest type of bonding to comprehend is
ionic
bonding
. This involves loss of an electron from
one atom, and its transfer to another, with bonding
resulting from the strong electrostatic attraction. For
this ionic bonding, the electron transfer is from an
atom with a low
ionization potential
to an atom with
high electron affinity, and the atomic objective is to
mimic for each atom the nearest noble gas electronic
configuration.
Let us consider sodium and chlorine. Sodium
(
1
s
2
2
s
2
2
p
6
3
s
1
)
has one electron more than neon
(
1
s
2
2
s
2
2
p
6
)
, and chlorine
(
1
s
2
2
s
2
2
p
6
3
s
2
3
p
5
)
has
one electron less than the noble gas argon
(
1
s
2
2
s
2
2
p
6
3
s
2
3
p
6
)
. Chlorine has high
electronegativity
(see
Section 2.7) and acquires one electron to become a
1
s
1
H
1
s
2
He
1
s
2
2
s
1
Li
1
s
2
2
s
2
Be
1
s
2
2
s
2
2
p
1
or 1
s
2
2
s
2
2
p
x
1
B
or 1
s
2
2
s
2
2
p
x
1
2
p
y
1
C
1
s
2
2
s
2
2
p
2
or 1
s
2
2
s
2
2
p
x
1
2
p
y
1
2
p
z
1
N
1
s
2
2
s
2
2
p
3
or 1
s
2
2
s
2
2
p
x
2
2
p
y
1
2
p
z
1
1
s
2
2
s
2
2
p
4
O
or 1
s
2
2
s
2
2
p
x
2
2
p
y
2
2
p
z
1
1
s
2
2
s
2
2
p
5
F
or 1
s
2
2
s
2
2
p
x
2
2
p
y
2
2
p
z
2
1
s
2
2
s
2
2
p
6
Ne
