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Atomic Masses

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Atomic Masses

Test Friday!

- 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

- 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

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

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?

- Let’s do the More Average Atomic Mass worksheet.