A simple relativistic Bohr atom
739
for example for Fm (fermium,
Z
=
100), with energies of 136.02 keV and 161.614 keV,
respectively.
A final example, revealing the significance of the relativistic treatment, is related to the
radius of the atom. The size (diameter) of the nucleus is in the range of 1.6 fm (fm
=
10
−
15
m)
(for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium).
These dimensions are much smaller than the size of the atom itself by a factor of about 23 000
(uranium) to about 145 000 (hydrogen). When we apply the relativistic model the circular
orbit of the uranium hydrogen-like ion has a radius of 426 fm. In this case the electron is now
much closer to the nucleus and the finite size of the nucleus should be accounted for.
The expressions found for the velocity, radius and energy of the hydrogen-like atoms
suggest that, in order not to contradict special relativity (speed of electrons higher than speed
of light) or not to achieve square roots of negative numbers (implying imaginary radius and
energy), the quantity
Zα
should be smaller than unity. Hence, the relativistic version of
Bohr’s model for the hydrogen-like ions suggests that
Z <
1
/α
∼
137. This relation sets an
upper limit on the atomic number of atoms in nature. This is in agreement with present-day
experimental facts, since the largest observed atom has an atomic number of 118 (Ununoctium,
discovered in 2006)
1
.
Finally, it becomes obvious from the energy expression that the relativistic model predicts
a very different spectrum for the hydrogen-like ions with very heavy nuclei (large
Z
). Today
it is an experimental fact that these deviations from the classical model are found even for not
very large ions, such as, for example, gallium with atomic number
Z
=
31 [
11
].
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