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Thomson observed a beam of charged particles traveling toward the positive electrode in a cathode ray tube. He deflected these beams using electric and magnetic fields, concluding the particles were negatively charged, low-mass constituents of atoms. These particles were later named electrons, fundamentally changing our understanding of atomic structure.

Rutherford directed positively charged alpha particles at thin gold foil. Most passed through undeflected, indicating atoms contain mostly empty space. A few scattered at large angles or bounced back, revealing a tiny, dense, positively charged nucleus at the atom's center containing most of the atom's mass.

Atoms contain protons, neutrons, and electrons. Protons carry a charge of +1, neutrons have no charge, and electrons carry a charge of −1. The charge of a proton and electron are equal in magnitude but opposite in sign, allowing atoms to maintain electrical neutrality when electron and proton numbers are equal.

Protons and neutrons have nearly identical masses of approximately 1 amu each, while electrons have a negligible mass of about 0.00055 amu. It would take roughly 1800 electrons to equal the mass of one proton. The atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

The atomic mass unit (amu) is a standard unit for expressing the extremely small masses of subatomic particles. One amu equals 1.66 × 10−27 kg, or one-twelfth the mass of a carbon-12 atom. This unit is more practical than kilograms for describing atomic-scale masses and charges.

Protons and neutrons are located in the nucleus at the atom's center, which contains most of the atom's mass. Electrons orbit around the nucleus in the surrounding empty space. The nucleus is roughly 100,000 times smaller in diameter than the entire atom, yet contains nearly all of its mass.

Chadwick discovered neutrons as uncharged particles in the nucleus with mass similar to protons. This explained why isotopes of the same element have different masses: they contain different numbers of neutrons. However, isotopes remain chemically identical because they have the same number of protons, which determines the element's identity.