Living Reference Work Entry

Encyclopedia of Geochemistry

Part of the series Encyclopedia of Earth Sciences Series pp 1-3

Date: Latest Version

Atomic Number, Mass Number, and Isotopes

  • Russell S. HarmonAffiliated withNorth Carolina State University Email author 


The primary building blocks of atoms are protons, neutrons, and electrons. It is convenient to describe the composition of an atom in terms of the number of protons and neutrons in its nucleus (Figure 1). The term atomic number, conventionally denoted by the symbol Z, indicates number of protons present in the nucleus of an atom, which is also equal to the number of electrons in an uncharged atom. The number of neutrons is represented by the neutron number (N). Because the mass of these nuclear particles is each approximately equal to one unified atomic mass unit (u), the sum of the protons plus neutrons is designated as the mass number (A). The mass of the electron is more than 1800 times smaller than the proton mass and, therefore, can be neglected in calculating the mass number. For any element, the mass number is equal to the atomic weight rounded off to the nearest integer value.
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