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Hydrogen-1 Hydrogen-2 Hydrogen-3
TABLE 8.1

1p 1p 1p Selected atomic weights calculated from mass and
0n 1n 2n abundance of isotopes
Stable Mass of Isotope Atomic
1
1 H 2 1 H 3 1 H Isotopes Compared to C-12 Abundance (%) Weight
1

H 1.007 99.985
(deuterium) (tritium) 1 2

H 2.0141 0.015 1.0079

1
9
Mass Be 9.01218 100. 9.01218

4
number 14
N 14.00307 99.63

7
15

1 Chemical N 15.00011 0.37 14.0067
7
16
1 H symbol O 15.99491 99.759

8
17
O

8 16.99914 0.037
18

O 17.00016 0.204 15.9994
Atomic 8
19
number F 18.9984 100. 18.9984
9
20

FIGURE 8.7 The three isotopes of hydrogen have the same Ne 19.99244 90.92
10
21

number of protons but different numbers of neutrons. Hydrogen-1 Ne 20.99395 0.257
10
is the most common isotope. Hydrogen-2, with an additional neu- Ne 21.99138 8.82 20.179
22

10
tron, is named deuterium, and hydrogen-3 is called tritium. 22

Al 26.9815 100. 26.9815
13
of an element must have the same number of protons in their
nuclei, the number of neutrons may vary. Atoms of an element
that have different numbers of neutrons are called isotopes. The sum of the number of protons and neutrons in a
There are three isotopes of hydrogen illustrated in Figure 8.7. All nucleus of an atom is called the mass number of that atom.
three isotopes have the same number of protons and electrons, Mass numbers are used to identify isotopes. A hydrogen atom
but one isotope has no neutrons, one isotope has one neutron with 1 proton and 1 neutron has a mass number of 1 + 1, or 2,
(deuterium), and one isotope has two neutrons (tritium). and is referred to as hydrogen-2. A hydrogen atom with 1 pro-
An atom is very tiny, and it is impossible to find the ton and 2 neutrons has a mass number of 1 + 2, or 3, and
mass of a given atom. It is possible, however, to compare the is referred to as hydrogen-3. Using symbols, hydrogen-3 is
mass of one atom to another. The mass of any atom is com- written as
pared to the mass of an atom of a particular isotope of carbon. 3
H
1
This particular carbon isotope is assigned a mass of exactly
. . .
12.00 units called atomic mass units (u). Since this isotope where H is the chemical symbol for hydrogen, the subscript to
is defined to be exactly 12 u, it can have an infinite number of the bottom left is the atomic number, and the superscript to the
significant figures. This isotope, called carbon-12, provides the top left is the mass number.
standard to which the masses of all other isotopes are com- How are the electrons moving around the nucleus? It
pared. The relative mass of any isotope is based on the mass of might occur to you, as it did to Rutherford and others, that
a carbon-12 isotope. an atom might be similar to a miniature solar system. In this
The relative mass of the hydrogen isotope without a neu- analogy, the nucleus is in the role of the Sun, electrons in the
tron is 1.007 when compared to carbon-12. The relative mass of role of moving planets in their orbits, and electrical attractions
the hydrogen isotope with 1 neutron is 2.0141 when compared between the nucleus and electrons in the role of gravitational
to carbon-12. Elements occur in nature as a mixture of isotopes, attraction. There are, however, big problems with this idea. If
and the contribution of each is calculated in the atomic weight. electrons were moving in circular orbits, they would continu-
Atomic weight for the atoms of an element is an average of the ally change their direction of travel and would therefore be
isotopes based on their mass compared to carbon-12, and their accelerating. According to the Maxwell model of electro magnetic
relative abundance in nature. Of all the hydrogen isotopes, for radiation, an accelerating electric charge emits electromagnetic
example, 99.985 percent occur as the isotope without a neutron radiation such as light. If an electron gave off light, it would
and 0.015 percent are the isotope with one neutron (the other lose energy. The energy loss would mean that the electron
isotope is not considered because it is radioactive). The frac- could not maintain its orbit, and it would be pulled into the
tional part of occurrence is multiplied by the relative atomic oppositely charged nucleus. The atom would collapse as elec-
mass for each isotope, and the results are summed to obtain the trons spiraled into the nucleus. Since atoms do not collapse
atomic weight. Table 8.1 gives the atomic weight of hydrogen as like this, there is a significant problem with the solar system
1.0079 as a result of this calculation. model of the atom.
8-5 CHAPTER 8 Atoms and Periodic Properties 207