Chemistry, Third edition

3 · INSIDE THE ATOM

This tiny mass is exactly equal to one-twelfth of the mass of one atom of carbon (isotope

mass number 12). This means that the mass of one atom of^126 C is exactly 12 on the

atomic mass scale. This is written as m(^126 C)12 u.

The alpha particle is also included in Table 3.1. This particle is a helium nucleus

and may also be symbolized as^42 He^2 . Now try Exercise 3B.

Isotopes

Introduction

Atoms with the same number of protonsbut with different numbers of neutronsare

calledisotopes. In other words, isotopes of an element possess the same atomic

number but different mass numbers. For example, consider two isotopes of car-

bon,^126 C and^136 C. Carrying out a ‘particle count’ for atoms of both isotopes, we

have

Count^136 C^126 C

number of electrons 6 6

number of protons 6 6

number of neutrons 7 6

In summary, both isotopes of carbon contain six protons and six electrons, but

atoms of carbon-13 are heavier because they each contain an extra neutron. (Try

Exercise 3C.)

3.2

32

Numbers of protons, neutrons and electrons

How many protons, neutrons and electrons do neutral atoms of each of the following

isotopes contain?

(i)The two main naturally occurring isotopes of uranium:

(^23952) U (^23982) U
(ii)The three isotopes of hydrogen:
(^11) H (normal hydrogen or protium)
2
1 H (heavy hydrogen or deuterium)
(^31) H (super heavy hydrogen or tritium)
Exercise 3C
Masses of particles
Answer these questions
using Table 3.1:
i(i)How much heavier is the
proton than the
electron?
(ii)How much heavier is the
alpha particle than the
proton?
Exercise 3B
Key points about isotopes
1.Isotopes have identical chemical reactions, although their compounds may
have slightly differing physical properties (such as boiling point, or density). For
example, hydrogen gas composed of normal hydrogen atoms (^11 H 2 ) and hydrogen
gas composed of deuterium atoms (^21 H 2 ) both display identical chemical reactions
and both burn in oxygen making normal water (^11 H 2 O) and heavy water (^21 H 2 O),
respectively. However, the density and boiling point of^21 H 2 is greater than that of
1
1 H 2 , and that of
2
1 H 2 O is greater than that of
1
1 H 2 O.
2.Some isotopes are radioactive(discussed further in Chapter 21).