1.6 - Where an atom's mass is concentrated
Most of an atom’s mass is concentrated in its tiny nucleus. Use particle masses to explain why occupying more space does not mean containing more mass.
Why most mass is in the nucleus
The nucleus contains protons and, in most atoms, neutrons. Each has relative mass about 1. Electrons surround it in shells, but each electron has relative mass only about 1/1840.
Consequently, the particles in the nucleus account for nearly all of the atom’s mass. Electrons have a small, nonzero mass, so “most” is more precise than “all”. Even an ordinary hydrogen atom, whose nucleus has one proton and no neutron, has most of its mass in the nucleus because its electron is so much lighter.
Mass and size tell different stories
The nucleus occupies a tiny fraction of an atom’s volume. The surrounding electron region accounts for most of the atom’s size but very little of its mass. The atom’s mass is therefore not spread evenly through its volume.
Think of two different questions: “How far does the atom extend?” concerns size; “Which particles contribute most to its mass?” concerns the masses and locations of its particles.