Biomedical Engineering Reference
In-Depth Information
3
The Nucleus and Nuclear Radiation
3.1
Nuclear Structure
The nucleus of an atom of atomic number
Z
and mass number
A
consists of
Z
pro-
tons and
N
A
-
Z
neutrons. The atomic masses of all individual atoms are nearly
integers, and
A
gives the total number of
nucleons
(i.e., protons and neutrons) in
the nucleus. A species of atom, characterized by its nuclear constitution—its values
of
Z
and
A
(or
N
)—is called a
nuclide
. It is conveniently designated by writing the
appropriate chemical symbol with a subscript giving
Z
and superscript giving
A
.
For example,
1
H, and
1
H, and
23
92
U are nuclides. Nuclides of an element that have
different
A
(or
N
) are called
isotopes
; nuclides having the same number of neutrons
are called
isotones
; for example,
20
82
Pb and
20
80
Hg are isotones with
N
= 124
. Hydro-
gen has three isotopes,
1
H,
1
H, and
1
H, all of which occur naturally. Deuterium,
2
=
1
H, is stable; tritium,
1
H, is radioactive. Fluorine has only a single naturally oc-
curring isotope,
1
9
F; all of its other isotopes are man-made, radioactive, and short
lived. The measured atomic weights of the elements reflect the relative abundances
of the isotopes found in nature, as the next example illustrates.
Example
Chlorine is found to have two naturally occurring isotopes:
3
17
Cl, which is 76% abun-
dant, and
3
17
Cl, which is 24% abundant. The atomic weights of the two isotopes
are 34.97 and 36.97. Show that this isotopic composition accounts for the observed
atomic weight of the element.
Solution
Taking the weighted average of the atomic weights of the two isotopes, we find for the
atomic weight of Cl, 0.76
×
34.97 + 0.24
×
36.97
=
35.45, as observed. (See periodic
table in back of book.)
Since the electron configuration of the different isotopes of an element is the
same, isotopes cannot be separated chemically. The existence of isotopes does
cause a very slight perturbation in atomic energy levels, leading to an observed
“isotope shift” in some spectral lines. In addition, the different nuclear spins of
