Atomnumret

Atomic number, denoted Z, is the number of protons in the nucleus of an atom. It is the defining property of an element: all atoms with the same Z belong to the same element and share the same electronic structure in their neutral state. The nuclear charge set by Z determines the arrangement of electrons around the nucleus and thus largely governs the element’s chemical behavior.

In the periodic table, elements are ordered by increasing Z. The mass number A, by contrast, equals the total number of protons and neutrons in the nucleus, so atoms with the same Z can have different A and are called isotopes.

Z can be determined experimentally through nuclear measurements or by relating it to observed X-ray frequencies via Moseley’s law. In notation, Z is written as a property of an element, and in neutral atoms the number of electrons equals Z. In ions, the electron count changes to Z minus the net charge.

Understanding Z is essential for predicting electron configurations, bonding, and periodic trends such as ionization energy, atomic radius, and electronegativity. It also distinguishes chemical identity from other nuclear properties like neutron number and mass.

Historically, the concept of atomic number clarified the organization of the periodic table. Henry Moseley showed that increasing nuclear charge, rather than atomic mass alone, better accounts for the observed properties of elements, leading to the modern arrangement of elements by Z.