Chemistry Reference
In-Depth Information
Hund's rule shows the order of occupation of degenerate orbitals—that is, the
pairing of electrons on the orbitals will not take place until after each orbital contains
one electron. These (single) electrons have the same spin; 1 that is, they are parallel.
The filling of orbitals with electrons starts with the lowest energy. The orbitals
of the p, d, and f subshells have the same energy for the corresponding subshell.
Therefore, these are filled with one electron first, and then receive the pair. On one
orbital the two electrons are antiparallel; their magnets are oriented in opposite
directions (↑↓). Using these rules, the distribution of electrons on the orbitals of
an element can be defined easily. For example, chlorine has 17 electrons. They will
be placed on the following orbitals: 1s
2
, 2s
2
, 2p
6
, 3s
2
, 3p
5
. The following schematic
representation is obtained:
↑↓
↑↓
↑↓
↑↓
↑↓
↑↓
↑↓
↑↓
↑
1s 2s 2p
x
2p
y
2p
z
3s 3p
x
3p
y
3p
z
In this context, we note that an energy level can only have 2, 8, 18, or 32 electrons
(Table 2.2).
The distances between the energy levels, or subshells, are not uniform. The filling
of orbitals with electrons is done from the lowest energy level, and the next one starts
to fill only after the previous level is full.
From the structure of polyelectronic atoms, we would expect that electrons would
fill the orbitals according to the following order: 1s < 2s < 2p < 3s < 3p < 3d < 4s <
4p < 4d < 4f and so on. In fact, data has shown that the energies of the orbitals do not
increase progressively according in this order. The order of filling of the orbitals is
dependent on the shielding effects (Z*) and the electronic penetration of the orbitals.
Due to their charges, the electrons are attracted by the nucleus and repel each
other. Thus, polyelectronic atoms have three types of energy:
•
the kinetic energy of the electron motion around the nucleus,
•
the energy of attraction of the electron to the nucleus, and
•
the energy of rejection from the other electrons.
The electrons in lower (inner) layers more strongly shield the electrons in the outer
layers than do the electrons on the same subshell or orbital shield, or they reject each
TABLE 2.2
The Number of Electrons in Layers and Sublayers
n
l
m
l
Orbitals
Electrons
K
1
0
0
s
2
L
2
0, 1
0, −1, 0, +1
s, p
2 + 6 = 8
M
3
0, 1, 2
0, −1, 0, +1; −2, −1, 0, +1, +2
s, p, d
2 + 6 + 10 = 18
N
4
0, 1, 2, 3
0, −1, 0, +1; −2, −1, 0, +1, +2;
0, −1, 0, +1; −3, −2, −1, 0, +1, +2, +3
s, p, d, f
2 + 6 + 10 + 14 = 32
