Question

When you look up the atomic mass of carbon in a table such as the one on the inside front cover of this book, you will find that its value is not 12.00 amu but 12.01 amu. The reason for the difference is that most naturally occurring elements (including carbon) have more than one isotope. This means that when we measure the atomic mass of an element, we must generally settle for the average mass of the naturally occurring mixture of isotopes.

• For example, the natural abundances of carbon-12 and carbon-13 are 98.90 percent and 1.10 percent, respectively.
• The atomic mass of carbon-13 has been determined to be 13.00335 amu.
• Thus, the average atomic mass of carbon can be calculated as follows:

Answer 1

The atomic mass of carbon on the periodic table is 12.011 amu, which is calculated as a weighted average of its isotopes, primarily carbon-12 and carbon-13. This accounts for the slight difference from the standard mass of carbon-12, which is exactly 12 amu. The molar mass of carbon is identical to its average atomic mass in grams per mole.

Step-by-step explanation:

The periodic table lists the atomic mass of elements, including carbon, as a weighted average of the masses of their naturally occurring isotopes. Carbon's atomic mass is noted as 12.011 amu because it is predominantly made up of carbon-12, with a natural abundance of 98.90 percent and an atomic mass of 12 amu.

It also contains carbon-13, with a natural abundance of 1.10 percent and an atomic mass of 13.003355 amu.

Thus, the average atomic mass of carbon is calculated by multiplying the natural abundances of these isotopes by their respective atomic masses and summing these products.

Carbon-12 and carbon-13 have mass numbers of twelve and thirteen, respectively. This difference in their mass numbers is due to the different number of neutrons they contain. Carbon-12 has six neutrons while carbon-13 has seven neutrons.

The average molar mass of carbon, which is the mass of 6.022 × 1023 carbon atoms (one mole), is therefore 12.011 g/mol, as both carbon-12 and carbon-13 are considered in the calculation of carbon's average atomic mass.