atomic number

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atomic number

atomic number, often represented by the symbol Z, the number of protons in the nucleus of an atom, as well as the number of electrons in the neutral atom. Atoms with the same atomic number make up a chemical element. Atomic numbers were first assigned to the elements c.1913 by H. G. J. Moseley; he arranged the elements in an order based on certain characteristics of their X-ray spectra and then numbered them accordingly. The elements are now arranged in the periodic table in the order of their atomic numbers. Mendeleev's periodic law was originally based on atomic weights. See mass number.
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Atomic number

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, &ngr;, 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 &ngr; for all elements studied, using 13 as the atomic number for aluminum and 79 for that of gold.

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. See Isotope, Radioactivity

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 Mass number

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

atomic number

Symbol: Z . The number of protons in the nucleus of an atom. This is equal to the number of electrons orbiting the nucleus in a neutral atom. The isotopes of an element have the same atomic number but different mass numbers.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Atomic Number

 

the number indicating the order of a chemical element in the periodic system of elements of D. I. Mendeleev. The atomic number is equal to the number of protons in the atomic nucleus, which in turn is equal to the number of electrons in the electron shell of the corresponding neutral atom. The atomic number is symbolized by Z. The nuclear charge is equal to Ze, where e is the positive elementary electrical charge equal in absolute value to the charge of the electron.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

atomic number

[ə′täm·ik ′nəm·bər]
(nuclear physics)
The number of protons in an atomic nucleus. Also known as proton number.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

atomic number

the number of protons in the nucleus of an atom of an element.
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005