By Lesley Barker – Updated Aug 30 2022
Atoms are the fundamental units of matter, each uniquely defined by its electron, proton, and neutron configuration. This structure determines an element’s identity.
Electrons are virtually massless, negatively charged particles that orbit the nucleus in discrete energy shells. The maximum occupancy of each shell follows the 2n² rule, and the spatial distribution is often described as an electron cloud or wavefunction.
Protons, residing in the nucleus, carry a positive charge equal in magnitude to that of an electron. Their attraction to electrons maintains charge neutrality in neutral atoms. The number of protons—the atomic number—uniquely identifies an element.
Neutrons, also located in the nucleus, have no net charge but provide the strong nuclear force that binds protons together. As Anthony Carpi of the City University of New York explains, neutrons act like “glue,” preventing the repulsive forces between positively charged protons from destabilizing the nucleus.
In a neutral atom the counts of protons and electrons are identical. When this balance is disturbed, the resulting charged species are called ions, not atoms.
The atomic number (Z) counts protons, and, in a neutral atom, also electrons. Atomic weight (or mass number) approximates the sum of protons and neutrons and can be found in the Periodic Table of Elements.
Both electrons and protons are charged sub‑atomic particles; their charge signs differ, and electrons are effectively weightless while protons have measurable mass. Electrons orbit the nucleus, attracted to the positive charge of the protons.
Historical milestones: J. J. Thomson received the 1906 Nobel Prize for discovering the electron, and Ernest Rutherford identified the proton in 1918.
For further reading, see the Periodic Table of Elements.
