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Chapter 3: Problem 2

What is the mass (in amu) of a carbon-12 atom? Why is the atomic mass of carbon listed as 12.01 amu in the table on the inside front cover of this book?

Short Answer

Expert verified
The atomic mass of a carbon-12 atom is exactly 12 amu as per definition of amu. However, the atomic mass of carbon listed on the inside front cover of the book as 12.01 amu because this value represents a weighted average of the masses of all naturally-occurring isotopes of carbon.

Step by step solution

01

Understanding Atomic Mass Unit (amu)

The atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom, which makes the atomic mass of a Carbon-12 atom is exactly 12 amu.
02

Explain the variance in Atomic Mass

All elements, including carbon, have multiple isotopes i.e., atoms with different numbers of neutrons and thus different masses. Carbon has three isotopes, Carbon-12, Carbon-13, and Carbon-14. When we state the atomic mass, we are looking at the weighted average of these isotopes. As not all atoms are Carbon-12, but a mix of the different isotopes, the average has a small fractional part, resulting in 12.01 amu. This fractional value represents a weighted average of the masses of individual carbon atoms from samples on Earth, taking into consideration the percentages of naturally-occurring isotopes.

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