More at 1 picometer
The nucleus is where nearly all of the mass of an atom is located.
The proton and neutron are each about 1800 times more massive than an
electron. An atom is mostly empty space. If the nucleus were one millimeter
in diameter, an atom would be larger than 100 meters in diameter. This
table shows the actual mass of particles in grams and atomic mass units
The standard of atomic weights is based on the carbon atom. Carbon12 is defined to have a mass of exactly 12 amu. Carbon12 contains 6 protons, 6 neutrons, and 12 electrons for a total combined mass of 12.10 amu. The extra 0.10 amu was converted into energy during the process of fusing hydrogen to helium to carbon in the core of a star.
Copyright © 2016 by Bruce Bryson