Chemistry Reference
In-Depth Information
•
m is the mass of the electron (9.1*10
−31
kg), and
•
e is the charge of the electron (1.6*10
−19
C).
The radius of the hydrogen atom, known as the Bohr radius, is used as a unit of
atomic radius. For the other atoms, the calculation of the atomic radius of the free
atom involves the relationship:
r=
(n
)
2
eff
H
(2.13)
r
Z
eff
where n
eff
is the effective principal quantum number, and Z
eff
is the effective nuclear
charge.
These measures are relative sizes of atoms because they cannot be measured accu-
rately as might be done for some tangible objects such as a ball or wheel. The relative
atomic radii according to the corresponding atomic numbers are shown in Figure 2.6.
It is very interesting that the atomic radii decrease within a period according to
the atomic number and increase within groups. The resulting tendency is an increase
in the size of atoms from the right to the left of the periodic table, and from the top
to the bottom (of groups) (
Figure 2.7
)
.
Although with increasing atomic number in period the number of electrons and
number of occupied electrons increase, the atomic radius decreases. This results
from the increase in the effective nuclear charge that produces a “contraction” in
periods of the orbitals toward the nucleus. In those cases where the outer electron
layer is the same valence layer, the increasing attraction of electrons by the nucleus
leads to a decrease in atomic radius. In groups, the atomic radius increases due to the
increasing number of electronic layers.
A special case is that of lanthanides where the 4f subshell is occupied progressively
before the filling of the 5d orbital from cerium (Z = 58) to lutetium (Z = 71) (
Table 2.9
).
The f orbitals have weak shielding and penetration properties so that the electrons are
3.5
Cs
3
Rb
K
2.5
Ac
Na
2
Li
1.5
Rn
1
L
Br
Cl
0.5
F
0
1
5
9
13
17
21
25
29
33
37
49
Atomic Number, Z
41
45
53
57
61
65
69
73
77
81
85
89
93
FIGURE 2.6
The variation of the atomic radius according to the atomic number.
