By Neha Tripathi Updated Mar 24, 2022
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The structure of an atom can be visualized as a miniature solar system. Electrons orbit the dense nucleus much like planets orbit the sun. While gravity holds planets together, the forces that bind an atom are electromagnetic attraction and the strong nuclear force.
The nucleus sits at the center of every atom and contains protons and neutrons. The number of protons—its atomic number—defines the element; helium always has two protons, carbon six, and so on. Isotopes of the same element arise when the neutron count varies; most hydrogen atoms have no neutrons, but rare forms carry one or two. The strong nuclear force, a fundamental interaction, keeps protons and neutrons bound within the nucleus.
Protons are the only positively charged subatomic particles in an atom. Each carries a charge of +1.6022 × 10⁻¹⁹ coulombs, equal in magnitude to a electron’s negative charge. With a mass of 1.67 × 10⁻²⁷ kilograms, a proton is roughly 1,837 times heavier than an electron and nearly as heavy as a neutron.
Electrons, denoted by the symbol e⁻, are the sole negatively charged particles in an atom. Their mass is a mere 1.1 × 10⁻³¹ kilograms. Electrons occupy discrete energy shells surrounding the nucleus; each shell has a limited capacity (2, 8, 18, 32, …) that determines the atom’s chemical behavior. Because the shells lie far from the nucleus, atoms consist of more than 99 % empty space.
Neutrons are neutral particles that share the nucleus with protons. All elements except hydrogen contain at least one neutron. A neutron’s mass is 1.6749 × 10⁻²⁷ kilograms, almost identical to a proton’s. In radioactive decay, a neutron may be emitted and wander freely for roughly 15 minutes before transforming into a proton and an electron.
