3. X Mass number Atomic # protons + # neutrons mass number # protons X
Mass
number
Atomic
# protons
+ # neutrons
mass number
# protons

4. Isotopes
Atoms of the same element can have different numbers of neutrons
This gives them different mass numbers.
They have the same atomic numbers.
They look, act and react the same.
C vs C-14

6. Naming Isotopes Put the mass number after the name of the element
Examples:
Carbon-12 and Carbon-14
Uranium-235 and Uranium-238

9. Which isotopes are represented in this sketch?

10. Calculations of Average Atomic Mass
Why is the mass number on a periodic table never a nice whole number?

11. Mass Number vs AverageAtomic Mass
The Mass Number is specific to one isotope.
The Average Atomic Mass is the calculated mass based on the abundance of each naturally occurring isotope of that element.
The Average Atomic Mass is shown on Periodic Tables. It is the most accurate number that should be used in calculations.

12. Average Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
oxygen-16,
oxygen-17 and
oxygen-18
We are more concerned with average atomic mass, based on the abundance of each element in nature.
We don’t use grams because the numbers would be too small.
California WEB

13. Atomic Mass Unit (amu or u) This unit is used instead of grams.
Atomic Mass Unit
(amu or u)
This unit is used instead of grams.
It is 1/12 of the mass of a carbon-12 atom.

14. Your book shows this a different way
Experts show that chlorine is a mixture of 75.77% Cl-35 and 24.33% of Cl-37. If the precise molar mass of Cl-35 is
g and Cl – 37 is grams what is the average molar mass of the chlorine atoms in the mixture?

15. Assume 100 moles of atoms are used in mixture
Calculate the number of moles of each isotope
Calculate the mass of each isotope
Add the masses together to get the total mass of the mixture

16. Total mass of mixture = mass of Cl-35 + Cl-37 = 35.453g
# moles of Cl-35 = x 100 mol = mol of Cl-35
75.77 mol X g/mol = g of Cl-35
# moles of Cl-37 = x 100 mol = mol
24.23 mol X g/mol = g of Cl-37
Total mass of mixture = mass of Cl-35 + Cl-37 = g