Question

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

Short answer

The mass of carbon-12 is exactly 12 amu; the atomic mass 12.01 amu accounts for isotopic variations.

Step-by-step solution

Understanding Carbon-12

Carbon-12 is a specific isotope of carbon, which means it has a particular number of protons and neutrons. Carbon-12 has 6 protons and 6 neutrons. The definition of an atomic mass unit (amu) bases the carbon-12 atom as exactly 12 amu, since 1 amu is one twelfth of the mass of a carbon-12 atom.

Clarifying Atomic Mass

The atomic mass listed in a periodic table is not for a single isotope. Instead, it's a weighted average of all the isotopes of that element found in nature. Carbon exists primarily as carbon-12, but also as other isotopes like carbon-13.

Calculating Average Atomic Mass

To find the average atomic mass, we consider the relative abundance of each isotope. Since carbon-12 is more predominant, its influence is significant, while carbon-13 contributes less to the average, both adjusted by their natural abundance.

Result Interpretation

The periodic table lists carbon's atomic mass as 12.01 amu. This is slightly higher than 12 because it includes contributions from heavier isotopes like carbon-13, even though they're less abundant than carbon-12.

Key concepts

carbon-12

Carbon-12 is an important isotope of carbon and serves as the standard for atomic mass units (amu). An isotope is a version of an element that has the same number of protons but a different number of neutrons. For carbon-12, this means it has 6 protons and 6 neutrons. This total gives it an atomic mass of exactly 12 amu.
This precision helps scientists as all atomic masses are calibrated against carbon-12.
By definition, one amu is defined as one twelfth the mass of a carbon-12 atom, making it the benchmark for measuring atomic masses of other isotopes and elements.

isotopes

Isotopes are different forms of the same element's atoms. They have the same number of protons but vary in the number of neutrons.
For instance, while carbon-12 has 6 neutrons, other carbon isotopes have different numbers. This difference doesn't change the chemical behavior of the element but slightly alters its mass.
Isotopes are critical for understanding atomic masses because elements usually exist in nature as mixtures of different isotopes.

• Carbon-12: Most prevalent with 6 protons and 6 neutrons.
• Carbon-13: Less common, with 6 protons and 7 neutrons.

average atomic mass

The average atomic mass of an element is a weighted average of all its naturally occurring isotopes. It considers the relative abundance and mass of each isotope. To calculate it, you multiply each isotope's mass by its natural abundance (expressed as a decimal), sum these values, and round as necessary.
For example, for carbon:

• Carbon-12 ( mass = 12 amu) is more abundant, greatly influencing the average.
• Carbon-13 ( mass = 13 amu) contributes less due to its rarity.

Hence, even a small fraction of heavier isotopes causes the average atomic mass to be slightly over 12, listed as 12.01 amu on the periodic table.

carbon-13

Carbon-13 is a less common isotope of carbon, having 6 protons and 7 neutrons. It plays an important role in calculations of the average atomic mass due to its heavier mass compared to carbon-12.
Despite its low abundance in nature, carbon-13's presence still affects the atomic mass calculation, slightly raising the final average.

• Mass = 13 amu: One amu more than carbon-12.
• Lower abundance: Less impact, but measurable.

The inclusion of carbon-13 helps explain why the periodic table lists carbon's atomic mass as about 12.01 amu rather than a perfect 12.