What Is the Charge of a Proton and How Is It Measured?
A proton has a charge of +1 elementary charge, written +e. In coulombs, its charge is exactly +1.602176634 × 10⁻¹⁹ C under the International System of Units (SI). The positive sign follows the convention for electric charge; an electron has the same charge magnitude but the opposite sign.
The elementary charge, e, is defined in the SI as exactly 1.602176634 × 10⁻¹⁹ coulombs. Since a proton carries one positive elementary charge, its charge is +e. The NIST CODATA Fundamental Physical Constants lists the defined value.
A coulomb is a much larger unit than the charge of a single proton. The charge of one proton is about one ten-billionth of a billionth of a coulomb. At the atomic scale, a proton’s positive charge attracts negatively charged electrons and contributes to an atom’s net electric charge.
Proton charge and atomic charge
An atom has no net electric charge when it has equal numbers of protons and electrons: each proton contributes +e, and each electron contributes −e. Their charges cancel.
If an atom gains or loses electrons, it becomes an ion. Its net charge depends on the difference between its numbers of protons and electrons. The proton’s charge remains +e; a change in electron count changes the atom’s overall charge.
Example: A sodium ion, Na⁺, has one fewer electron than a neutral sodium atom. Its net charge is +1e, or +1.602176634 × 10⁻¹⁹ C—the same net charge as one proton, even though the ion contains many protons.
What “exact” means
The SI value of e is exact by definition, so the proton’s charge in coulombs is also exact in SI units. This does not mean every measurement involving a proton is exact. Measurements can have uncertainty because of the instruments or methods used; the defined value of e provides an exact conversion between elementary charges and coulombs.
Key Takeaways
- A proton’s charge is +1 elementary charge, written +e.
- In SI units, this is exactly +1.602176634 × 10⁻¹⁹ C.
- An electron has the same charge magnitude but the opposite sign.
- An atom’s net charge depends on its numbers of protons and electrons.
