1. Isoptopes and Average Atomic Mass
Chemistry 11
Ms. McGrath

2. Atomic Mass
Recall:
The mass number, A, tells us the sum of protons and neutrons in the nucleus of an atom.
The mass of an atom is expressed in atomic mass units (µ).
Example:
An oxygen that has 8 protons and 8 neutrons in its nucleus has a mass number of 16.

3. Atomic Mass
Recall:
All neutral atoms of the same element has the same number of protons and electrons. However, the number of neutrons can vary.
Therefore, atoms of the same element do not necessarily have the same mass number.

4. oxygen-16 ( ), oxygen-17 ( ), oxygen-18 ( )
Isotopes
Most oxygen atoms have a mass number of 16 (A = 16)
There are, however, two other naturally occurring forms of oxygen. One of these has 9 neutrons, so A = 17 and the other has 10 neutrons, so A = 18.
These forms are referred to as isotopes.
These three isotopes of oxygen are called:
oxygen-16 ( ), oxygen-17 ( ), oxygen-18 ( )

5. Isotopes - Oxygen
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes of oxygen:

6. Isotopic Abundance
The relative amount in which each isotope is present in an element is the isotopic abundance. Abundance can be expressed as a percent (100%) or as a decimal (1.00).
For example, magnesium has three natural occurring isotopes.
It is made up of 79% magnesium-24, 10% magnesium-25 and
11% magnesium-26.

7. Average Mass and the Periodic Table
The average atomic mass on the periodic table is the average of the masses of all the element’s isotopes. The abundance of each isotope of the element must be taken into account when calculating the average atomic mass.

8. Calculating the Average Mass
Natural occurring silver exists as two isotopes. From the mass of each isotope and the isotope abundance listed below, calculate the average atomic mass of silver.
Isotope
Atomic Mass (µ)
Relative abundance (%) Ag
106.9
51.8
108.9
48.2