mass number: the mass of one individual atom (protons + neutrons). Always a whole number.
abundance: the percentage of atoms of an element that are one specific isotope.
average atomic mass: the estimated average of of the mass numbers of all of the atoms of a particular element in the universe.
Analogy: average atomic mass works the same way as class average on a test.
1. Multiply each score times the number of students who got it.
2. Add up the number for each score to get the total points.
3. Divide the total by the number of students to get class average.
The abundance of each of the isotopes of carbon is:
· carbon-12: 98.9%
· carbon-13: 1.1%
· carbon-14: 0.000 001%
What is the average atomic mass of carbon?
1. Multiply abundance x mass # for each isotope
2. Add up the number from each isotope to get the total
3. Divide by 100 (because the abundances add up to 100%)
4. Check that your answer is in between the mass number of the smallest isotope and the mass number of the largest one.
1. Multiply abundance x mass # for each isotope
98.9 x 12 = 1186.8
1.1 x 13 = 14.3
0.000 001 x 14 = 0.000 014
2. Add up the number from each isotope to get the total
1186.8 + 14.3+ 0.000 014 = 1201.100 014
(round to 1201.1)
3. Divide by 100 (because the abundances add up to 100%)
1201.1 ÷ 100 = 12.011
4. Check that your answer is in between the mass number of the smallest isotope and the mass number of the largest one.
Yes, 12.011 is between 12 and 14.