1. ISOTOPES AVERAGE ATOMIC MASS

2. ISOTOPES Atoms of the same element with different mass numbers
Atoms with the same number of protons, but different numbers of neutrons.
ISOTOPES OF CHLORINE
35Cl 37Cl
Chlorine Chlorine - 37

3. FACTS ABOUT ISOTOPES
Almost each element is found in the Universe as a mixture of isotopes
Every isotope is found in nature in a fixed percentage called Isotopic Abundance
Example: Magnesium can be found as a mixture of three isotopes 79% Magnesium-24, 10% Magnesium-25 and 11% Magnesium-26

4. FACTS ABOUT ISOTOPES These percentages are different for each element
Each element has a particular number of isotopes
The isotope notation includes the name or symbol of the element followed by the atomic mass.
Example: Magnesium-25 or Mg-25

5. FACTS ABOUT ISOTOPES
The existence of isotopes explains the fact that atomic mass is a decimal for most elements
Isotopes of one element have the same chemical behaviour but slightly different physical properties
Example: boiling point or melting point

6. FACTS ABOUT ISOTOPES
They have lots of real life applications e.g. medical
There are almost 2000 isotopes found
The isotopes of radioactive elements are called Radioisotopes

7. LEARNING CHECK
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______

8. AVERAGE ATOMIC MASS Where: Aav = average atomic mass
Atomic mass is the weighted AVERAGE ATOMIC of all the atomic masses of the isotopes of that atom correlated with their isotopic abundances.
Where:
Aav = average atomic mass
(%) = isotopic abundance

9. PRACTICE ONE
Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine.
Calculate the average atomic mass for Chlorine.

10. PRACTICE TWO
Assume that the element 108Uno is synthesized and that the sample contains 25.2% 272Uno (271.4 amu), 30.8% 273Uno (272.3 amu) and 44.0% 275Uno (274.2 amu).
Calculate the average atomic mass of Uno.

11. CALCULATING ISOTOPIC ABUNDANCE
Steps:
Assign variables for the isotopic abundances; x and y
Write the equation for isotopic abundances:
x + y = 1
Write the equation for the Average atomic mass:
AAV = A1X1 + A2X2
Set both equation as a linear system and solve for x and y by substitution

12. PRACTICE THREE Boron exists as two naturally occurring isotopes:
B-10.01a.m u and B-11.01a.m.u.
Calculate the isotopic abundance of each isotope of Boron.

13. PRACTICE FOUR
Silver (Atomic weight ) has two naturally-occurring isotopes with isotopic weights of and
What is the percentage abundance of each isotope in Silver?