A LITTLE LIGHTBULB MOMENT
General chemistry: atoms, bonds and stoichiometry
Mass number counts protons plus neutrons
Sign in to bookmark this concept and add private notes →
Add the nucleus roommates: protons and neutrons
The idea behind the cue
Mass number is the total number of protons plus neutrons in one atom’s nucleus. It refers to a specific atom or isotope, not the periodic table’s average atomic mass.
Practice this flashcard →Make it memorable
Short & simple
Add the nucleus roommates: protons and neutrons
Acronym
PNM — Purple Noodles Mumble.
Story
A backpack label reads “mass.” The atom stuffs in the heavy nucleus residents: protons and neutrons. Electrons hover outside with too little mass to matter much in this count.
Connect the pieces
- P (Purple) → Protons: Mass number includes the protons in one atom's nucleus.
- N (Noodles) → Neutrons: Mass number also includes the neutrons in that same nucleus.
- M (Mumble) → Mass: So mass number is the total of protons plus neutrons for a single atom or isotope, not the periodic table average atomic mass.
- Purple · Protons: Mass number includes the protons in one atom's nucleus.
- Noodles · Neutrons: Mass number also includes the neutrons in that same nucleus.
- Mumble · Mass: So mass number is the total of protons plus neutrons for a single atom or isotope, not the periodic table average atomic mass.
Know the limits
Use for a single atom or isotope, not the periodic table average atomic mass.
Further reading · OpenStax science and mathematics textbooks ↗Check your understanding
Which quantity equals protons plus neutrons in one atom? A. Atomic number B. Mass number C. Valence number D. Ionic charge
A. B. Mass number
B. C. Valence number
C. D. Ionic charge
D. A. Atomic number
Reveal explanation
B. Mass number
Mass number is the count of nucleons: protons plus neutrons. Atomic number is only protons.