1. Isotopes
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Objectives:
5.2 Identify the advantages and disadvantages of using isotopes in industry, medical science, basic research, and in the environment.
5.3 Analyze the irregularities in the progression of the atomic masses of the elements in the periodic table.

2. Irregularities Of Mass In The Periodic Table
Elements do not have their atomic masses as whole numbers.
The atomic mass of an element is the weighted average of all the different isotopes of that element
This is also known as Relative Mass.

3. Calculating Relative Mass
To calculate relative mass do the following:
STEP # 1:
You need to know the relative abundance of each isotope (expressed as a percent %)
and the atomic mass of each isotope.
Use the formula below to solve for relative mass.
STEP # 2:
Relative Mass =
(amu X % abundance) + (amu X % abundance)
100

4. EXAMPLE # 1
Using the information given below about Lithium, find its relative mass.
amu % Abundance
Li u %
Li u %
Lithium
Mass = (amu X % abundance) + (amu X % abundance)
100
Relative Mass = (6u X 7.42) + (7 X 92.58)
100
Mass = 6.93u

5. EXAMPLE # 2
Using the information given below about oxygen, find its relative mass.
amu % Abundance
O u %
O u %
O u %
Oxygen
= (16u X ) + (17 X 0.038) + (18 X 0.200)
100
Relative Mass =

6. Step # 1 EXAMPLE # 3 Step # 2 Isotope Atomic Mass % Abundance C-12
Find % abundance of second isotope…. 100% = ?
= 1.11%
EXAMPLE # 3
The atomic mass (relative mass) of carbon is amu. The chart below shows data on the two isotopes of carbon. What is the atomic mass of the second isotope of carbon?
Use the formula to solve for the atomic mass (working backwards)
Step # 2
Relative Mass = (amu X %abun.) + (amu X %abun.)
100
= (12u X 98.89) + (? X 1.11%)
100
Atomic mass =
Isotope
Atomic Mass
% Abundance
C-12
98.89%
C-? ?
1.11%

7. EXAMPLE # 4 Step # 1 Si-28 99.762% Si-29 0.038% Si-30 0.200%
TRICKY!
EXAMPLE # 4
Silicon (Si) has 3 naturally occurring isotopes. One isotope has 14 neutrons and a % abundance of 92.2, another isotope has 15 neutrons and a % abundance of 4.70, and finally the last isotope has 16 neutrons and a % abundance of 3.09.
What is the approximate average atomic mass of Silicon?
Step # 1
Get the atomic mass for each isotope….
% Abundance
Si %
Si %
Si %
NOTE: The question only gave the neutrons….so you add them to the protons!

8. Step # 2 Relative Mass =28.106u Si-28 92.2% Si-29 4.70% Si-30 3.09%
Plug information into formula and solve
% Abundance
Si %
Si %
Si %
= (28u X 92.2) + (29u X 4.70) + (30u X 3.09)
100
Relative Mass =28.106u

9. Key Points to Remember Calculating relative mass: Relative Mass =
amu x % abundance + amu x % abundance
100
Atomic mass