Atoms are the fundamental building blocks of all visible matter, yet they are mostly empty space. Every atom has an incredibly dense central nucleus containing positive protons and neutral neutrons. These heavy central particles are collectively known as nucleons by physicists and chemists.
The strong nuclear force binds them tightly together to overcome the electrostatic repulsion between protons. Almost all of an atom’s mass is concentrated directly inside this tiny central nucleus. A large cloud of negatively charged electrons surrounds the nucleus and dictates the total atomic volume.
Electrons do not travel in perfect circles, but instead occupy specific probability regions called orbitals. Complex rules of quantum mechanics govern exactly how these three-dimensional orbitals are shaped. In a neutral atom, the total number of electrons perfectly matches the number of protons.
Every element is defined entirely by its proton count, which is known as the atomic number. Atoms with the same proton count but different neutron counts are classified as isotopes. Isotopes behave identically in chemical reactions but possess distinct atomic masses.
Atoms try to achieve thermodynamic stability by completely filling their outermost valence electron shells. They do this by sharing, losing, or gaining outer electrons to form molecules and stable compounds.