More about isotopes

Atomic mass vs average atomic mass or atomic weight

Atomic Mass

The mass of a specific isotope of an element expressed in atomic mass units (amu)

The atomic mass is the total mass of the protons, neutrons and electrons in the atom

Atomic mass = mass of an atom

This is what we refer to in our isotope expression, eg.  

Average Atomic Mass (also called Atomic Weight)

This is the number that you see in the periodic table.

This is the weighted average of the all of the isotopes of Hydrogen

1

H

1.008

Atomic Number

Symbol of element

Atomic Weight

What is a Weighted Average?

The isotopes of an element do not occur with equal frequency or amounts

For example, the relative abundances for the three carbon isotopes are:

carbon-12 98.9% carbon-13 1.1%, carbon-14 <001%.

The "average" mass for the atoms of an element is dictated by the most abundant or common isotope

The atomic weights in the periodic table = weighted averages

Tabulated atomic weight value doesn't match any actual atom, but is closer to the weight for the most common isotope

We’ll follow the steps of how to calculate a weighted average next….

Calculating Weighted Average

Carbon-12 atomic mass = 12 amu; abundance = 98.9%

Carbon-13 atomic mass = 13 amu; abundance = 1.1%

Carbon-14 atomic mass = 14 amu; abundance = 0.001%

weighted average = ( decimal fraction A) mass A + ( decimal fraction B) mass B

Atomic weight = (0.989 * 12) + (0.011 * 13) + (0.00001 * 14) = 12.011

6

C

12.011

Average Atomic Mass WorksheetHomework - I will post answers on website Monday. You will have to show ALL of your work.