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## Atomic Masses

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**Atomic Masses**Test Friday!**Atomic Mass is an average (decimal)**• An average of known isotopes for an element • The atomic mass of an element is closest to the most common isotope found in nature • So, since Carbon’s atomic mass is closest to 12, Carbon-12 would be the most common isotope found in nature**19**K 39.0983 Isotopes and atomic mass • Potassium • How many protons? • How many neutrons? 19 20 potassium atoms: 93.25% will have 20 neutrons, 6.7302% will have 22 neutrons, 0.0117% will have 21 neutrons**How is that weight calculated?**potassium atoms: 93.25% will have 20 neutrons, 6.7302% will have 22 neutrons, 0.0117% will have 21 neutrons All will have 19 protons, so…. (.9325*20) + (.067302*22) + (.000117*21) = 39.0983 amu a weighted average 93.25% will weigh 39 amu 6.7302% will weigh 41 amu 0.0117% will weigh 40 amu**Remember!**100% in decimal form = 1 50% = .5 35% = .35 10 = .1 1% = .01 .1% = .001 If in doubt, divide the percentage by 100!**Approximately 75% of the chlorine atoms found innature have**a mass of 35. The other 25% have a massof 37. What should we report as the atomic weight forchlorine? • [(.75)(35)] + [(.25)(37)] • = 26.25 + 9.25 • = 35.5 • The KEY is to convert the percentages to decimals correctly!**1. Suppose that there were two isotopes of Sodium.28% of the**naturally occurring sodium atoms had amass of 22, and 72% atoms had a mass of 23. Whatwould the average atomic weight of sodium be?**2. Suppose that there were two natural isotopes ofCopper.**80% of the atoms had a mass of 63, and 20%of the atoms had a mass of 65. What would thataverage atomic weight of copper be?**3. Suppose that a new element (E) were discovered that**existed as three natural isotopes. 25% of the atoms had a mass of 278, 38% had a mass of 281, and the remainder had a mass of 285. What would be listed as the atomic weight of this element?**Practice!**• Let’s do the More Average Atomic Mass worksheet.