Biomedical Engineering Reference
In-Depth Information
The various combinations of neutrons and protons that may exist in nature are il-
lustrated by examining the composition of the nuclei of the three types of hydrogen atom
(hydrogen, deuterium, and tritium). Although hydrogen has a nucleus consisting of a single
proton, the combination of one proton and one neutron exists as a single particle called a
demtron
and is the nucleus of the atom called heavy hydrogen, or deuterium. Extending this
concept further, the combination of two protons and two neutrons forms a stable particle—
the alpha particle—that in nature exists as the nucleus of the helium atom. Alpha radiation
emitted from radioactive substances consists essentially of a stream of such particles.
Tritium, the third atom of hydrogen, on the other hand, has a nucleus consisting of only
one proton and two neutrons.
These three types of hydrogen are examples of atoms whose nuclei have the same
number of protons (i.e., have the same atomic number (Z)) and at the same time may have
a different number of neutrons and, thereby, a different atomic mass (A). Atoms exhibiting
this characteristic were given the name
isotope
(from the Greek, meaning “same place”). The
term
has been widely used to refer to any atom, particularly a radioactive one. How-
ever, current usage favors the word
isotope
to refer to a particular combination of neutrons
and protons. Thus, isotopes are nuclides that have the same atomic number. All elements
with an atomic number (Z) greater than 83 and/or atomic mass (A) greater than 209 are
radioactive—that is, they decay spontaneously into other elements, and this decay causes
the emission of active particles.
In order to specifically designate each individual atom, certain symbols have been devel-
oped. In the process of reading the literature in this field, it is therefore necessary to become
somewhat familiar with them. In the United States, it was the practice to place the atomic
number as a subscript before and the atomic mass as a superscript after the chemical
symbol of the atom (
53
I
131
). Since the chemical symbol itself also specifies the atomic num-
ber, one often omits it and simply writes I
131
. In Europe, on the other hand, it was custom-
arily written as a superscript prior to the chemical symbol (
131
I). In an effort to achieve
international standards, it was agreed in 1964 that the atomic mass should be placed as a
superscript preceding the chemical symbol (
131
I). When superscripts are not used, a more
literal form of designation, such as cobalt 60, is commonly used. Referring to the helium
atom, one simply writes
4
He, where 4 is equal to the atomic weight (because of the aggre-
gation of protons and neutrons in the nucleus).
Since the mass of an individual nuclear particle is very small and, when expressed in
grams, involves the use of unwieldy negative exponents, a system of atomic mass units
(amu) has been developed that uses carbon 12 (
1
6
nuclide
C) as a reference atom. The arbitrary
value of 12 mass units has been assigned to carbon-12. The masses of all other atoms are
based on a unit, that is
1
12
the mass of the carbon-12 atom. The mass of the lightest isotope
of hydrogen is thus approximately 1 amu.
The mass in grams of an isotope numerically equal to its atomic mass is called a gram-
atomic mass. A gram-atomic mass of a substance is also referred to as a mole or equivalent
mass of the substance. Since gram-atomic masses have magnitudes proportional to the
actual masses of the individual atoms, it follows that one mole of any substance contains
a definite number of atoms. The number of atoms in one gram-atomic mass is given by
Avogadro's number (N
A
) and is equal to 6.023
10
23
.
