Atomic mass, also known as atomic weight, isn't calculated for a single atom in the same way we might calculate the mass of a macroscopic object. Instead, it represents the weighted average mass of all isotopes of an element. This is because most elements exist as a mixture of isotopes – atoms with the same number of protons but differing numbers of neutrons.
Calculating the Atomic Mass of an Individual Isotope
For a single isotope, the atomic mass is simply the sum of the masses of its protons and neutrons. As noted by Khan Academy: "Together, the number of protons and the number of neutrons determine an element's mass number: mass number = protons + neutrons." The mass of electrons is negligible in this calculation.
- Example: Carbon-12 has 6 protons and 6 neutrons. Its atomic mass is approximately 12 atomic mass units (amu).
Calculating the Average Atomic Mass of an Element
To calculate the average atomic mass of an element, we need to consider the abundance of each isotope. This is a weighted average, taking into account the relative proportions of each isotope present in nature. The formula is:
Average Atomic Mass = (Isotope 1 Abundance × Isotope 1 Mass) + (Isotope 2 Abundance × Isotope 2 Mass) + ...
- Example: Chlorine has two main isotopes: Chlorine-35 (approximately 75% abundance) and Chlorine-37 (approximately 25% abundance). To calculate the average atomic mass:
Average Atomic Mass = (0.75 × 35 amu) + (0.25 × 37 amu) = 35.5 amu
This average atomic mass is what you typically find listed on the periodic table.
Where to Find the Information
The information needed for this calculation—isotope masses and their abundances—is usually found in:
- Periodic tables: The atomic mass listed is the average atomic mass.
- Chemistry textbooks and reference books: These often provide detailed isotope information.
- Online databases: Numerous databases contain extensive atomic mass data.
The number of protons and neutrons are used for determining the mass of a single isotope; however, the average atomic mass, as seen on the periodic table, is calculated by weighing the contribution of each isotope based on its natural abundance. This weighted average reflects the composition of an element as it typically occurs in nature.
