1. Isotopes and Average Atomic Mass

2. Isotopes The number of protons define the type of atom:
6 protons is carbon
1 proton is hydrogen
8 protons is oxygen
92 protons is uranium
However the number of neutrons can vary within a given type of atom

3. Hydrogen Isotopes There are three main isotopes of hydrogen
Hydrogen – 1p, 0n, 1e
Deuterium – 1p, 1n, 1e
Tritium – 1p, 2n, 1e

4. Relative Mass of an Atom
The mass of an atom is expressed in atomic mass units (amu)
Atomic mass is determined based upon carbon-12 as a standard with a mass exactly equal to 12amu
Using this, the relative mass of every atom on the periodic table can be determined (i.e. oxygen 16 would have a mass of 16amu)

5. Isotopic Abundance
Because elements as a combination of different isotopes, it is useful to consider the isotopic abundance
For example, magnesium is composed of:
Magnesium-24 – 79%
Magnesium-25 – 10%
Magnesium-26 – 11%
Regardless of where you find magnesium, it will be composed of these percentages

6. Average Atomic Mass and the Periodic Table
The average atomic mass that appears on the periodic table is the average of the mass of all the element’s isotopes
Therefore, even carbon, which is used as the standard, does not have an average atomic mass of 12 but rather due to the presence of carbon-14
Carbon-14 is about 1.1% of all carbon while carbon-12 is the remaining 98.1%

7. Average Atomic Mass Calculation

8. Average Atomic Mass
Naturally occuring silver exists as two isotopes. From the mass of each isotope and the isotopic abundance listed below, calculate the average atomic mass of silver
Isotope
Atomic mass (u)
Abundance (%)
Silver-107
106.9
51.8
Silver-109
108.9
48.2

9. Average Atomic Mass

10. Why do we need to know Average atomic mass?
To calculate formula mass and molecular mass:
3 Steps to calculate the formula mass/molecular mass
Above all Double Check to make sure you have the correct formula
Count the atoms of each element
Multiply the number of atoms by the elements atomic mass (round mass to hundredths)
Add all the masses together

11. Formula Mass
This is the mass of one molecule and the unit is atomic mass units (amu)
This is a very, very small value so it is not very useful in the laboratory.
We use moles of molecules, which is a group of molecules.