Mass Number vs. Atomic Mass: Key Difference Explained
Mass Number is the total count of protons and neutrons in one atom’s nucleus. Atomic Mass is the weighted average mass of all isotopes of an element, measured in unified atomic mass units (u).
Students see “mass” in both names and assume they’re synonyms, but chemists and radiologists treat them differently. A surgeon ordering an isotope scan needs the Mass Number for dosage, while a lab tech balancing equations uses Atomic Mass from the periodic table.
Key Differences
Mass Number is always a whole number (protons + neutrons). Atomic Mass is a decimal because it averages the naturally occurring isotopes, weighted by abundance.
Which One Should You Choose?
Use Mass Number when naming isotopes like Carbon-14. Use Atomic Mass when calculating molar mass or stoichiometry in lab work.
Examples and Daily Life
Iodine-131 has Mass Number 131, crucial for thyroid scans. The periodic table lists Iodine’s Atomic Mass as 126.90 u, the value you need to prepare a 0.1 M solution.
Can Atomic Mass equal Mass Number?
Only for a single isotope; natural samples contain mixtures, so Atomic Mass is almost always a decimal.
Why is Carbon-12 special?
It’s the reference isotope: exactly 12 u Atomic Mass, making it the calibration standard for the unified scale.