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

Isotopes and atomic mass

• Potassium • How many protons? • How many neutrons? 20

potassium atoms:93.25% will have 20 neutrons,6.7302% will have 22 neutrons,0.0117% will have 21 neutrons

19

K19

39.0983

How is that weight calculated?

(.9325*20) + (.067302*22) + (.000117*21) =39.0983 amu

a weighted average

potassium atoms:93.25% will have 20 neutrons,6.7302% will have 22 neutrons,0.0117% will have 21 neutronsAll will have 19 protons, so….

93.25% will weigh 39 amu6.7302% will weigh 41 amu0.0117% will weigh 40 amu

Remember!

100% in decimal form = 150% = .5

35% = .3510 = .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 for

chlorine?

• [(.75)(35)] + [(.25)(37)] • = 26.25 + 9.25

• = 35.5The 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. What

would 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.