Chemistry, Third edition

3 · INSIDE THE ATOM

Atomic mass of elements and of isotopes

The mass of one atom of an isotope on the atomic mass scale is called its isotopic

massm. Some isotopic masses are listed in Table 3.2. Remember that the mass of

atoms is defined with reference to a ‘standard atom’, i.e. one atom of carbon-12 for

whichm(^126 C)12.0000 u.

Notice that isotopic masses (in atomic mass units) are very nearly equal in size

to the mass number of an isotope. This is because the mass of the electrons, pro-

tons and neutrons are approximately 0, 1 and 1, respectively, on the atomic mass

scale. Note also that the units of mandAare not the same – Ais unitless.

If more than one isotope of an element exists in nature, the average massof

one atom of that element will be weighted towards the mass of the most abundant

isotope. The average mass of one atom of an element in nature is called the atomic

massof that element, symbolized m(element). The atomic mass of an element is

worked out using the equation

atomic mass 

(m 1 P 1 )(m 2 P 2 )(m 3 P 3 )...

(3.1)

100

wherem 1 ,m 2 andm 3 are the isotopic masses of isotopes 1, 2 and 3, and P 1 ,P 2 and

P 3 are the percentage natural abundances of those isotopes.

An example: chlorine

Chlorine atoms in nature consist of those with mass number 35 and those with mass

number 37. We start by taking the isotopic masses as equal to the mass numbers:

m(^3157 Cl)35 u

m(^3177 Cl)37 u

If there were equal numbers of both isotopes in nature, the atomic mass of chlorine

would be the average of 35 u and 37 u, i.e. 36 u. In doing this averaging, we are

really using equation (3.1) with P 1 50% and P 2 50%:

atomic mass of Cl 

(3550)(3750)

36 u

100

The actual abundances (Table 3.2) are 75.770% and 24.229%. Substituting these

values in equation (3.1) gives

m(Cl)

(3575.770)(3724.229)

35.48 u

100

Use of exact isotopic masses (Table 3.2) gives the accepted value for the atomic

mass of chlorine:

m(Cl)

(34.968975.770)(36.965924.22)

35.45 u

100

34

Working out atomic mass of oxygen

Using the isotopic masses and abundances shown in Table 3.2, confirm that the atomic

mass of oxygen is 16.00 u.

Exercise 3E