1. Isotopes Average Atomic Mass

2. Isotopes  Atoms of the same element (same atomic number) with different mass numbers  Atoms with the same number of protons, but different numbers of neutrons. Isotopes of chlorine 35 Cl 37 Cl17 Chlorine - 35 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 For example: Magnesium can be found as a mixture of three isotopes 79% Mg-24, 10% Mg-25 and 11% Mg-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. 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 (e.g. boiling point, melting point) They have lots of real life applications (medical field, agriculture,..) There are almost 2000 isotopes found The isotopes of radioactive elements are called Radioisotopes

6. Learning Check Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C 6 6 6 #p _______ _______ _______ #n _______ _______ _______ #e _______ _______ _______

7. Atomic Mass on the Periodic Table 11 Na 22.99 Atomic Number Symbol Atomic Mass

8. Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom correlated with their isotopic abundances. where: A av = average atomic mass (%) = isotopic abundance

9. Example 1: Average Atomic Mass Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine. Calculate the average atomic mass for Chlorine.

10. Example 2: Average Atomic Mass Assume that the element 108 Uno is synthesized and that the sample contains 25.2% 272 Uno (271.4 amu), 30.8% 273 Uno (272.3 amu) and 44.0% 275 Uno (274.2 amu). What is the value of the atomic weight?

11. Calculating Isotopic Abundance Steps: Assign variables for the isotopic abundances; x and y Write the eq. for isotopic abundances: x + y = 1 Write the eq. for the Average atomic mass: Set both eq. as a linear system and solve for x and y by substitution

12. Practice! 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! Silver (Atomic weight 107.868) has two naturally-occurring isotopes with isotopic weights of 106.90509 and 108.90470. What is the percentage abundance of each isotope in Silver?