atomic number

The number of protons in an atomic nucleus.

The number of elementary positive charges (protons) contained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom.

The concept of atomic number emerged from the work of G. Moseley, done in 1913–1914. He measured the wavelengths of the most energetic rays (K and L lines) produced by using the elements calcium to zinc as targets in an x-ray tube. The square root of the frequency, ν, of these x-rays increased by a constant amount in passing from one target to the next. These data, when extended, gave a linear plot of atomic number versus ν for all elements studied, using 13 as the atomic number for aluminum and 79 for that of gold. See also X-ray spectrometry.

Moseley's atomic numbers were quickly recognized as providing an accurate sequence of the elements, which the chemical atomic weights had sometimes failed to do. Additionally, the atomic number sequence indicated the positions of elements that had not yet been discovered.

The atomic number not only identifies the chemical properties of an element but facilitates the description of other aspects of atoms and nuclei. Thus, atoms with the same atomic number are isotopes and belong to the same element, while nuclear reactions may alter the atomic number.

When specifically written, the atomic number is placed as a subscript preceding the symbol of the element, while the mass number (A) precedes as a superscript, for example, ²⁷₁₃Al, ²³⁸₉₂U. See also Atomic structure and spectra; Element (chemistry); Mass number.

1. the number of electrons outside the nucleus of a neutral atom. n 2. the number of protons in the nucleus.

The number of protons in the nucleus of an atom; more correctly the number of unit charges of such a nucleus measured arithmetically in terms of the charge of an electron but with opposite sign. Such a figure equates with the number of protons under current theory, and equals the number of surrounding electrons under non-ionized conditions. The term was first used in 1865, but only effectively to number the elements sequentially, for placement in the periodic table; it was nearly 50 years later that its charge basis was realized, hence the scientific basis of the periodic table. The values of atomic number range from 1 for hydrogen and 2 for helium to 6 for carbon, 8 for oxygen, 26 for iron, 79 for gold, 92 for uranium, and up to 103 for lawrencium, all being strictly integers, with no gap. (The ‘atomic weight’ includes the chargeless neutrons plus the relatively minor mass of the electrons; for hydrogen this gives 1.008; generally atomic weight, correctly called relative atomic mass, is more than twice the atomic number.)

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The number of protons or electrons normally found in an atom of a given chemical element. The higher the atomic number, the heavier the atom is. In a neutral atom, the number of protons and electrons is the same. (See atomic weight and periodic table of the elements.)

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