Chemistry Reference
In-Depth Information
A proton is a positively charged subatomic particle with a mass of ap-
proximately 1 atomic mass unit (amu) and a charge of +1. It is the number
of protons in the nucleus of an atom, or the nuclear charge, that gives the
element its identity. For example, any atom containing only one proton in
its nucleus is considered an atom of the element hydrogen, whereas any
atom containing eight protons in its nucleus is considered an oxygen atom.
The number of protons in an atom is called the atomic number of that ele-
ment. So, we can say that the atomic number of hydrogen is 1, and the
atomic number of oxygen is 8. A single proton is often represented by the
symbol (p
+
) or ( ). Because a single proton is identical to the nucleus of
a form of hydrogen, the symbol (H
+
), which will we see often when we
study acids, also represents a proton.
A neutron ( ) is a neutrally charged subatomic particle, which, as does
the proton, has a mass of approximately 1 atomic mass unit (amu). When
we add the total number of protons and neutrons in the nucleus of an atom,
we get the atom's mass number. Because the neutron has no charge, it does
1
1
p
+
1
0
n
not affect the atomic
number and does not
alter the identity of
the element. For this
reason, it is possible
to have two atoms of
the same element
with differing mass
numbers, because
they have different
numbers of neutrons.
Atoms of the same
element with differ-
Three Isotopes of Hydrogen
Protium
A nucleus consist-
ing of one proton
plus one electron
in the cloud.
Deuterium
A nucleus consist-
ing of one proton
and one neutron,
plus one electron
in the cloud.
Tritium
A nucleus consist-
ing of one proton
and two neutrons,
plus one electron
in the cloud.
Figure 3-1b
ent masses are called isotopes. For example, there are three different iso-
topes of hydrogen, as shown in Figure 3-1b. Please note that the images in
this figure are not drawn anywhere near to scale.
The atomic mass unit (amu), which is represented with the symbol “u,”
is based on a particular isotope of carbon, called carbon-12. Carbon-12 is
considered to have a mass of exactly 12 u, and all of the other elemental
isotopes are measured relative to that isotope. The atomic masses, shown
on the periodic table, represent a weighted average of the masses of the
naturally occurring isotopes of each element. For example, some periodic
tables show an atomic mass of 1.00794 u for hydrogen, despite the fact that
no particular isotope of hydrogen has a mass number equal to that value.
