Chapter 1 The Early Experiments
1.9
SUBATOMIC PARTICLES, ISOTOPES, AND IONS
As summarized in Table 1.1, scientists have discovered that atoms are composed of three subatomic particles: protons, neutrons, and el
ectrons. The number of protons identifies the
nucleus, and the number of protons and neutrons determines its mass.
Atomic number, Z
, is the number of protons in t
he nucleus. It is the number that
characterizes an element. If the atomic number
of an element is known, then the identity of
the element is known and
vice versa
.^
Mass number, A
, is the sum of the number of protons
and the number of neutrons in the
nucleus. Because the mass of each of these par
ticles is nearly 1 amu, the mass number is
an integer that closely approx
imates the nuclear mass in atomic mass units (amu).
The atomic mass is sometimes included with the symbol of the element. For example, the notation
63 Cu (read copper-63) indicates that the mass number of Cu is 63. There is no
reason to include Z because it is implied in
the element’s symbol. Cu means Z = 29
because Cu always has 29 protons. Note that the number of neutrons is A minus Z. Thus, 63 Cu has 63 - 29 = 34 neutrons.*
Isotopes
are different forms of the same element that have the same atomic numbers
(Z) but different masses (A). C
onsequently, they differ only in the number of neutrons (A -
Z). For example:
35 Cl and
37 Cl, chlorine-35 and chlorine-37, are two isotopes of chlorine.
Both contain 17 protons because both are chlorine atoms. They differ in that
35 Cl contains
18 neutrons (35 - 17) while
37 Cl contains 20 neutrons (37 - 17). Naturally occurring
chlorine is 75.8%
35 Cl and 24.2%
37 Cl. It is this combination of the two that gives a mole
of chlorine a mass of 35.5 grams (the molar mass of chlorine).
Table 1.1 Subatomic particles
Particle Mass (amu) Charge
†^
electron 5.49 x 10
-4 -1
proton 1.00728 +1 neutron 1.00867 0
†
Charge at the atomic and molecular level is usually expressed as multiples of the fundamental unit
of charge as determined by
Millikan. Thus, the charge on the electron is designated as -1, which means that it carries a charge of (-1)(1.6 x 10
-19
C). The
charge on the proton is then (+1)(1.6 x 10
-19
C ).
* A and Z are sometimes shown in the atom’s symbol. Thus, the
element whose symbol is X and whose atomic number and mass are Z and A, respectively would be represented as follows:
AX Z
⇒
# protons
(# protons + # neutrons)
(^) X
This notation is redundant because the symbol (X) identifies the
element and its atomic number (Z).
However, the notation is used
when balancing nuclear equations.
Charge is not continuous; it comes in bundles
§ of 1.6x10
-19
C
. Electrons are bundles of
negative charge and protons are bundles of positiv
e charge. All charge is comprised of an
integral number of these bundles, and the net charge on a particle at the atomic and molecular level is usually given as the differe
nce between the positive and negative charge
carriers rather than the actual charge in coulombs.
§ Quantities that are not continuous
but come in discrete units are
said to be
quantized
, and the bundles that carry the units are
called quanta. Thus, electrons
and protons are quanta of negative
and positive electrical charge. In Chapter 2, we show that electromagnetic energy is also quantized.
charge on particle = number of protons – number of electrons
Atoms have no charge because the number of protons and electrons are equal. Thus, Al (Z=13) contains 13 protons, 13 electrons, a
nd no net charge. While the number of protons
in an atom is fixed, electrons can be added
or removed to produce charged particles called
ions
. Positively charged ions, which are called
cations,
are produced when electrons are
lost, and negatively charged ions, which are called
anions,
are produced when electrons
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