http://www.ck12.org Chapter 4. Atomic Structure
FIGURE 4.14
Mass spectrogram for carbon.element.
Example 4.2
Using the percent abundances for each carbon isotope as given above, calculate the weighted average for the atomic
mass of carbon.
Answer:
We calculate average atomic mass by taking the percent abundance of each isotope and multiplying this by the
atomic mass of the isotope.
(^12) C
0.989×12 = 11.868
(^13) C
0.011×13 = 0.143
Then, add these values together to get the relative atomic mass:
11.868 + 0.143 = 12.011
Therefore, the average atomic mass of carbon is calculated to be 12.011. This is the same number that is listed on
the periodic table.
Lesson Summary
- The atomic number (Z) of an element is equal to the number of protons in its nucleus.
- The mass number (A) of an element is equal to the sum of the protons and the neutrons in its nucleus.
- Isotopes are atoms of the same element that have a different mass number (A) but the same atomic number
(Z). - Isotopes have the same number of protons and electrons, but a different number of neutrons.
- The mass spectrometer measures the percent abundance of different isotopes in a given sample.
- The average atomic mass of an element can be calculated from the atomic mass and percent abundance of
each naturally occurring isotope.