Atomic mass vs. atomic number
Atomic number is the number of protons in an atom's nucleus and defines the element, while atomic mass reflects the total of protons and neutrons. Electrons contribute almost nothing to mass — a single electron weighs only about 0.00055 amu.
Atomic number and atomic mass describe two different things about an atom. The atomic number (Z) is the count of protons in the nucleus — it defines which element the atom is and, in a neutral atom, equals the number of electrons. The mass number (A) is the count of protons plus neutrons, and an element's atomic mass (the decimal value on the periodic table) is the weighted average of the masses of its naturally occurring isotopes.
A concrete example: carbon has atomic number 6, so every carbon atom has 6 protons. Carbon-12 has 6 neutrons (mass number 12) while carbon-13 has 7 (mass number 13); both are carbon because the proton count is unchanged. The periodic table lists carbon's atomic mass as about 12.01 amu because carbon-12 dominates naturally with a small fraction of carbon-13. Change the neutron count and you get an isotope; change the proton count and you get a different element entirely.
Mass lives almost entirely in the nucleus. A proton and a neutron each weigh about 1 amu (1 atomic mass unit, defined as one-twelfth the mass of a carbon-12 atom), while an electron weighs only about 0.00055 amu — roughly 1/1,836 the mass of a proton. That is why electrons are ignored when computing mass numbers, and why gaining or losing electrons changes an atom's charge but leaves its mass essentially unchanged.
The MCAT tests these distinctions in its atomic structure content: identifying protons, neutrons, and electrons from isotope notation, comparing isotopes, and knowing that nearly all of an atom's mass is nuclear. Keeping atomic number (protons, identity) separate from mass number and atomic mass (protons plus neutrons, weight) resolves most questions quickly.
Key takeaways
- Atomic number = number of protons; it defines the element and never changes for a given element.
- Mass number = protons + neutrons; atomic mass is the isotope-weighted average shown on the periodic table.
- Isotopes share an atomic number but differ in neutron count and therefore mass.
- An electron's mass is about 0.00055 amu — about 1/1,836 of a proton — so electrons are negligible in mass calculations.
- The MCAT tests isotope notation and the proton/neutron/electron breakdown of atoms and ions.