1. Average Atomic Mass Monday, September 21 st, 2015

2. The atomic mass on the periodic table of an element is a weighted average that represents all the atoms of that element. What is a weighted average? Hydrogen has 3 isotopes (3 forms of hydrogen in the universe): Hydrogen-1 [1 proton + 1 electron + 0 neutrons]= 1 amu Hydrogen-2 [1 proton + 1 electron + 1 neutrons]= 2 amu Hydrogen-3 [1 proton + 1 electron + 2 neutrons]= 3 amu then we would expect it to have an average atomic mass to be: Wait, that’s not right? The atomic mass is 1.01 amu. What’s going on?!! weighted average does not equal regular average

3. Why is it that a hydrogen atom with its 1 proton, 1 electron and 0 neutrons has an atomic mass of 1.01 amu? The mass of a proton is only 1 amu. Where does the other.01 amu come from? Is it because of the electron? 1/1840 =.0005434 ≠.01!!! Its not the electron its way too small!!

4. Calculating your grade in chemistry class... An analogy for Weighted Atomic Mass of an Element Class Grade Breakdown If the I received the following grades...what would my grade be?

5. Let’s calculate and see... Step 1: We want only a single grade so we must make the % assignments relative to a single grade (ie 1 grade) because 20% + 30% + 50% = 100% [the relative grade of each grade must add to 1 (or 100)] relative grade = % ÷ 100 Lab Midterm Final therefore (20% ÷ 100 )+ (30% ÷ 100 )+ (50%÷ 100) =1 and.2 +.3 +.5 = 1 [remember we are only giving a single grade]

6. Step 2: This means that the relative grade for each assignment are as follows... Relative grade value = (relative weight) x Grade Value so....2(93) = 18.6.3(85)= 25.5.5(82)= 41 Step 3: Calculate the grade My grade = lab rel. value + midterm rel. value + final rel. value So 18.6 + 25.5 + 41 = 85.1 I got a "B" in the class

7. The more atoms of a given type (isotopes), the more impact its mass will have on the periodic table atomic mass. The atomic mass on the periodic table must represent all the types of atoms for that element. It is a weighted average For Hydrogen: Hydrogen-1 which is 1 amu represents ~99.985% of the hydrogen atoms in the universe. Hydrogen-2 which is 2 amu is only ~.0150…% Hydrogen-3 which has 3 amu is negligible because it is radioactive and doesn’t hang around long enough to be measured... So the atomic mass is closest to Hydrogen –1 but larger because of Hydrogen 2.

8. Complete "Weighted Atomic Mass" Practice Problems #1-3 and put in your notebook

9. #1: A sample of element X contains 100 atoms with a mass of 12.00 and 10 atoms with a mass of 14.00. Calculate the average atomic mass (in amu) of element X. 100 - 12 amu 10 - 14 amu 100 = 0.91 110 10 = 0.09 110 0.91(12) = 10.92 amu 0.09(14) = 1.26 amu 12.18 amu

10. #2: In a sample of 400 lithium atoms, it is found that 30 atoms are lithium-6 (6.015 g/mol) and 370 atoms are lithium-7 (7.016 g/mol). Calculate the average atomic mass of lithium. 30 - 6.015 g/mol 370 - 7.016 g/mol 30 = 0.075 400 370 = 0.925 400 0.075(6.015 g/mol) = 0.451125 0.925(7.016 g/mol) = 6.4898 6.941 g/mol

11. #3: Magnesium has three naturally occurring isotopes. 78.70% of Magnesium atoms exist as Magnesium-24 (23.9850 g/mol), 10.03% exist as Magnesium-25 (24.9858 g/mol), and 11.17% exist as Magnesium-26 (25.9826 g/mol). What is the average atomic mass of Magnesium? 78.70% --> 0.7870 (23.9850 g/mol) = 18.876195 10.03% --> 0.1003 (24.9858 g/mol) = 2.5060757 11.17% --> 0.1117 (25.9826 g/mol) = 2.90225642 24.2834 g/mol

12. Extension: Copper has two naturally occuring isotopes. Cu-63 has an atomic mass of 62.9296 amu and an abundance of 69.15%. What is the atomic mass of the second isotope? What is its nuclear symbol? Copper's mass on the periodic table = 63.55 Isotope 1: 69.15% --> 0.6915 (62.9296) = 43.5158 63.55 - 43.5158 = 20.0342 1- 0.6915 = 0.3085 0.3085(mass) = 20.0342 mass = 20.0342 = 64.9407 amu 0.3085

13. How is the atomic mass of the periodic table determined?  Like a students grade the atomic mass is a weighted average You will simulate and practice the calculation of a weighted atomic mass using a substance not found naturally in nature. It has 3 isotopes. Your task will be to determine the atomic mass of this substance. Please note that the ‘experimental samples’ have been used by a number of students for years and MUST be returned to their vials in order to receive a treat