Standard Atomic Weights on Periodic Table (red numbers) are found through the terrestrial (Sol III) relative abundances of the masses (in amu) of the isotopes.

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Presentation transcript:

Standard Atomic Weights on Periodic Table (red numbers) are found through the terrestrial (Sol III) relative abundances of the masses (in amu) of the isotopes of that element. Atomic mass unit (1 amu) is defined as 1/12 the mass of the neutral Carbon-12 atom (includes the 6 protons, 6 neutrons, 6 electrons)

Average Atomic Mass If there are 3 naturally occurring isotopes of Neon including Ne-20 Ne-21 Ne-22. then the average mass of Neon atoms should be ___ amu. Why does the PT lists it as 20.1798 amu? Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

63Cu is 69.17% abundant 65Cu is 30.83% abundant Average Atomic Mass A weighted average atomic mass is calculated from both the masses of each isotope and the relative abundance (%) of each isotope. 63Cu is 69.17% abundant 65Cu is 30.83% abundant To which will the average mass be closer, 63 or 65?

Calculating Average Atomic Mass copper-63 has a mass of 62.93 amu and natural abundance of 69.17% copper-65 has a mass of 64.93 amu and has a natural abundance of 30.83% Calculating Average Atomic Mass Avg. Mass = (Mass1 x %frac) + (Mass2 x %frac) … = (62.93)(0.6917) + (64.93)(0.3083) = 43.53 + 20.02 = 63.55 amu

Neon-20 Neon -21 Neon-22 Which isotope is in greater relative abundance in natural samples of neon? because… the weighted average atomic mass of neon from the PT is 20.1798 amu. 90% - 20, <1% - 21, 9% - 22

=28.8 + 3.40 = 32.2 Avg. = (M1)(%) + (M2)(%) Quick Quiz! 1. If an unusually selected sample of sulfur contained 90.0% sulfur-32 (31.97 amu) and 10.0% sulfur-34 (33.97 amu), what would be its average atomic mass? 32.2 amu 32.9 amu 33.5 amu 34.2 amu Avg. = (M1)(%) + (M2)(%) = (31.97)(0.900) + (33.97)(0.100) =28.8 + 3.40 = 32.2

Changes on WorkSheet Question #2 has 3 isotopes Neon - 20 ( 19 Changes on WorkSheet Question #2 has 3 isotopes Neon - 20 ( 19.99 amu at 90.48%) Neon – 21 ( 20.99 amu at 0.27%) Note 2 Sig Fig on Abundance… Neon – 22 ( 21.99 amu at 9.25%) Question #3 Silicon – 28 ( 27.98 amu at 92.230%) Silicon – 29 ( 28.98 amu at 4.683%) Silicon – 30 ( 29.97 amu at 3.087%)