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
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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) =
a weighted average
93.25% will weigh 39 amu
6.7302% will weigh 41 amu
0.0117% will weigh 40 amu
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?
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?