Download Average Atomic Mass Key

ISOTOPES AND AVERAGE ATOMIC MASS Name _____________________ Elements come in a variety of isotopes, meaning they are made up of atoms with the same atomic number but different atomic masses. These atoms differ in the number of neutrons. The average atomic mass is the weighted average of all the isotopes of an element. Example:
A sample of cesium is 75%133Cs, 20% 132Cs and 5% 134Cs.
What is its average atomic mass?
Answer: 0.75 x 133 = 99.75
0.20 x 132 = 26.40
0.05 x 134 = _6.70_
Total = 132.85 amu = average atomic mass
Determine the average atomic mass of the following mixtures of isotopes. 1. 80% 127I, 17% 126I, 3% 128I ______________________________ AMM = 127(.80) + 126(.17)+128(.03) = 126.9 amu 2. 50% 197Au, 50% 198Au AAM = (0.50)(197)+(0.50)(198) = 197.5 amu ______________________________ 3. 15% 55Fe, 85% 56Fe AAM = (0.15)(55)+(0.85)(56) = 55.85 amu ______________________________ 4. 99% 1H, 0.8% 2H, 0.2% 3H AAM = (0.99)(1)+(0.008)(2)+(0.002)(3)= 1.012 amu ______________________________ 5. 95% 14N, 3% 15N, 2% 16N AAM = (0.95)(14)+(0.03)(15)+(0.02)(16)=14.07 amu___________________________ 6. 98% 12C, 2% 14C AAM = (0.98)(12)+( 0.02)(14)= 12.04 amu ______________________________ More Review Questions
1) Mass number is calculated by adding the total number of protons and neutrons in the nucleus of the
atom.
2) An element is identified by the number of protons in its nucleus.
3) The charge of a ion is calculated by subtracting the number of electrons from the number of protons
in the nucleus.
4) Changing the number of protons in an atom makes a different element.
5) Changing the number of electrons in a neutral atom makes an ion.
6) Changing the number of neutrons in the nucleus of an atom makes a different isotope of the element.
7) In the carbon-14, the number 14 represents the mass number.
8) How many protons, neutrons, and electrons are found in the nuclide shown below?
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26 p+, 30 n0, 23 e9) Explain how the weighted average used for calculating average atomic mass is different that a
regular average you would calculate in math class.
A standard average calculated in math class assumes that all values are equally abundant. However,
this is not true for isotopes. Some isotopes are more abundant than others A weighted average
reflects the abundances. The more abundant an isotope is, the greater effect its mass will have on the
average atomic mass.