Unit 2 Atomic Theory Structure Average Atomic Mass

Unit 2: Atomic Theory & Structure Average Atomic Mass

Mass of Atoms • One atomic mass unit (amu) is defined as 1/12 th the mass of a carbon-12 atom. 0 1 1

Average Atomic Mass If there are 2 naturally occurring isotopes of Neon including Ne-20 and Ne-22 then the average mass of Neon atoms should be 21 amu. ___ Why does the PT lists it as 20. 18 amu? Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

Average Atomic Mass A weighted average atomic mass is calculated from both the masses of each isotope and the relative abundance (%) of each isotope. 10 B is 19. 8% abundant 11 B is 80. 2% abundant To which will the average mass be closer, 10 or 11?

boron-10 has a mass of 10. 01 amu and natural abundance of 19. 8% boron-11 has a mass of 11. 01 amu and has a natural abundance of 80. 2% Calculating Average Atomic Mass Avg. Mass = (Mass 1)(%) + (Mass 2)(%) … = (10. 01)(0. 198) + (11. 01)(0. 802) = 10. 81 amu

Calculating Average Atomic Mass Element X has two natural isotopes: X-20 (mass of 20. 012 amu); 39. 91%. X-22 (mass of 22. 009 amu); 60. 09%. Calculate the average atomic mass. 20 X: (20. 012)(0. 3991) = 22 X: (22. 009)(0. 6009) = 7. 987 13. 225 21. 212 amu

Quick Quiz! 1. If an unusually selected sample of sulfur contained 90. 0% sulfur-32 and 10. 0% sulfur-34, what would be its average atomic mass? A. 32. 2 amu B. 32. 9 amu C. 33. 5 amu D. 34. 2 amu Avg. = (M 1)(%) + (M 2)(%) = (32)(0. 900) + (34)(0. 100) = 32. 2