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If you enlarged an atom until its nucleus were the size of a marble in the center zone of a football stadium, the electrons would be like specks of dust swirling around the highest bleachers.
Almost all of an atom is empty space, yet everything in the physical universe is constructed from just three fundamental subatomic particles: protons, neutrons, and electrons.
How do these particles distribute their mass and charge to hold the atom together?

The Anatomy of the Atom

Every atom consists of two distinct regions: a tiny, ultra-dense central core called the nucleus and a vast surrounding region known as the electron cloud.
How do these three particles compare when we measure their precise masses and electrical charges side by side?

Mass, Charge, and Discovery

Because subatomic particles are unimaginably small (a single proton has a mass of 1.673×10−27 kg1.673 \times 10^{-27}\text{ kg}), physicists measure them using the unified atomic mass unit (u\text{u}) and the elementary charge (e=1.602×10−19 Ce = 1.602 \times 10^{-19}\text{ C}).
In 1897, J.J. Thomson discovered the electron using cathode ray tubes; Ernest Rutherford identified the positively charged proton in 1917; and James Chadwick proved the existence of the neutral neutron in 1932.
Since nearly all the mass sits locked inside the nucleus, how do chemists express the exact particle counts for any specific atom?

Standard Nuclear Notation