An element's atomic mass is essentially the total mass of its protons and neutrons. We ignore the electrons because their mass is negligible compared to protons and neutrons. This mass is typically expressed in daltons (Da).
Understanding Atomic Mass
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Protons and Neutrons: The atomic mass is primarily determined by the number of protons and neutrons in the atom's nucleus. Each proton and neutron contributes approximately one dalton to the total mass. Khan Academy explains that the mass number is the sum of protons and neutrons (mass number = protons + neutrons).
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Isotopes and Weighted Average: Most elements exist as a mixture of isotopes—atoms with the same number of protons but a different number of neutrons. The atomic mass listed on the periodic table is a weighted average reflecting the abundance of each isotope. For example, chlorine has two main isotopes: chlorine-35 and chlorine-37. The atomic mass of chlorine is not exactly 35 or 37 but rather a weighted average reflecting the natural abundance of each isotope. Quora elaborates on this weighted average calculation.
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Atomic Mass Units (AMU or amu): The atomic mass unit (amu or Da) is defined as 1/12 the mass of a carbon-12 atom. This provides a standard unit for measuring the mass of atoms and molecules. TechTarget provides further information about the AMU.
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Calculating Atomic Mass: While the simplified calculation (protons + neutrons) gives an approximation, the actual atomic mass is determined experimentally using mass spectrometry techniques. These methods measure the mass of individual atoms and their relative abundance to precisely determine the weighted average atomic mass.
In Summary
The atomic mass of an element is a weighted average of the masses of its naturally occurring isotopes, primarily determined by the number of protons and neutrons in its nucleus. The mass is expressed in daltons (Da) or atomic mass units (amu).
