Visual Notes: Particle Masses & Atomic Weight
■ Masses of Fundamental Particles
Neutron (n)
m■ = 1.008664 u ≈ 939.565 MeV
Proton (p)
m■ = 1.007276 u ≈ 938.272 MeV
Electron (e)
m■ = 0.000548 u ≈ 0.511 MeV
➡■ Nucleon mass (p or n) is ~1830 times larger than electron mass.
■ Atomic Mass vs. Nuclear Mass
• Since m■ / m■ ≈ 1830, the atomic mass ≅ the nuclear mass (electrons negligible).
■ Nuclear Mass Formula
M■ucleus ≈ Zm■ + Nm■ ≈ (Z + N)u = A(u)
• Z = # of protons
• N = # of neutrons
• A = Z+N = mass number
■ Approximation for Atomic Weight
A(u) = M■tom ≈ ■(u)
■ Atomic weight (in u) ≈ Atomic number (A in u)
✔■ Good for 3–4 significant figures