Biomedical Engineering Reference
In-Depth Information
The mass of any atom in grams can thus be found by dividing the gram-atomic mass of
the isotope by Avogadro's number. For example, the mass of an atom of carbon-12 is its
gram-atomic mass (12 grams) divided by Avogadro's number (N
A
):
12 gm
10
23
gm
mass of
12
C
¼
10
23
¼
1
:
99
6
:
023
Since
12
C is equal to 12 atomic mass units (amu), the mass of 1 amu would be
1
12
the mass
of a carbon-12 atom.
10
23
gm
12
C atoms
1
:
99
=
1 amu
¼
12 amu
=
12
C atom
10
24
gm
1 amu
¼
1
:
66
The electrical charge carried by the electron is a fundamental property of matter, as is the
mass of a particle. Since this is the smallest amount of electricity that can exist, it is usually
expressed as a negative unit charge (-1). When expressing a unit charge in the metric system
of units (meters, kilograms, and seconds), one obtains the value 1.6
10
19
coulombs. The
charges carried by all other atomic particles are therefore some integral multiple of this value.
Consequently, it is impossible for a particle to have, for example, a charge equal to two and
one-half times that of the electron.
15.2.3 Atomic Structure and Emissions
As already mentioned, Bohr's model suggested the existence of an atomic structure anal-
ogous to the planetary system. In this system electrons rotated in discrete orbits or shells
around the nucleus, and the orbital diameters were determined by a quantum number (
)
having integer values. These orbits were then represented by K, L, M, and N, corresponding
to an increasing number of
n
a nomenclature still in use today. This model was further
refined in 1925 by Wolfgang Pauli in terms of quantum mechanical principles. Pauli's work
on atomic structure explained various observed phenomena, including the estimates for
binding energy of the electrons at various orbits of an atom. Pauli further observed that
an atom can be defined by four quantum numbers: (1)
n,
n
is the principal quantum number,
which is an integer and scalar quantity; (2)
is the angular momentum quantum number, a
vector quantity that has integral values ranging from 0 to
l
n
-1
;
(3) m
1
is the magnetic quantum
1; (4) m
s
is the spin magnetic quantum
number with integral values ranging from -1 to
þ
number, which has the values of
½ and -½. According to the Pauli exclusion principle,
no two electrons in an atom can have the same set of quantum numbers.
In an electrically neutral atom, the number of orbital electrons exactly balances the num-
ber of positive charges in the nucleus. The chemical properties of an atom are determined
by the orbital electrons, since they are predominantly responsible for molecular bonding,
light spectra, fluorescence, and phosphorescence. Electrons in the inner shells, on the other
hand, are more tightly bound and may be removed from their orbits only by considerable
energy such as by radiation interaction.
þ
