By Debra Durkee | Updated Mar 24, 2022
The mass of a single proton is exactly 1.672 621 636 × 10-27 kg, a value known to nine significant figures. In an atom’s nucleus, the total mass of protons is roughly equal to the mass contributed by neutrons. Since over 99 % of an atom’s mass resides in its nucleus, nearly half of the atom’s mass comes from protons alone. For context, a proton is about 1,860 times heavier than an electron.
Protons carry a positive elementary charge of +1 e, the fundamental unit of electric charge. This is exactly opposite the negative charge of an electron. The proton’s charge is a constant; it does not change with temperature, pressure, or time. Neutrons, by contrast, are electrically neutral.
Scientists determine the proton’s charge through several precision methods. The Josephson and von Klitzing constants link voltage and magnetic field to fundamental charge units. The Faraday method measures the charge transferred by an electric current; historically, this involved analyzing silver deposits after controlled electrochemical reactions. Although the Faraday constant has largely been replaced by the coulomb in modern units, it remains a staple in electrochemistry.
The proton’s positive charge is central to atomic stability. In hydrogen, the only atom with a single proton and no neutrons, the proton’s charge defines the hydrogen ion (H+). The balance between protons and electrons determines whether an atom is neutral or ionized, affecting its chemical behavior and interactions with electric or magnetic fields.
Ionization—removal of electrons—renders atoms unstable and charged. In environments like nuclear reactors or particle accelerators, ionized atoms (and the resulting free protons) can pose radiation hazards. However, in the upper atmosphere, natural ionization is typically harmless to biological systems.
