Chemical element

A pure substance whose atoms all have the same number of protons, defining its atomic number. Elements cannot be separated into simpler substances by chemical reactions.

Atomic number: The number of protons in an atom’s nucleus, which identifies its chemical element. It is the defining count that distinguishes one element from every other.
Nuclear reaction: A process that changes an atomic nucleus, potentially changing an atom’s element or isotope. Unlike chemical reactions, it can change the proton count defining an element.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the first minutes of the universe. It produced most primordial hydrogen and helium, with traces of other light nuclei.
Chemical formula: A symbolic representation of the elements and relative numbers of atoms in a substance. Element symbols show which kinds of atoms make up a compound.
Atom: The smallest unit of an element that retains its chemical properties, consisting of a nucleus and electrons. Elements are classified by the nuclei of their constituent atoms.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus, releasing particles or energy. Some decay modes change proton number and produce a different element.
Stellar nucleosynthesis: The creation of atomic nuclei through fusion and other nuclear processes in stars. It built many elements from lighter nuclei during stellar lifetimes.
Elemental analysis: The determination of which elements are present in a sample and, often, their amounts. It identifies elemental composition rather than merely naming a substance.
Isotope: A variant of an element whose atoms have different numbers of neutrons but the same number of protons. Isotopes show why neutron count can vary without changing elemental identity.
Nuclear transmutation: The conversion of one chemical element into another through a change in nuclear proton number. It is the direct nuclear route from one elemental identity to another.

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