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atomic number


n. (Symbol Z) (Abbr. at. no.)

The number of protons in an atomic nucleus.


 
 
Sci-Tech Encyclopedia: 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, 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.


 
Dental Dictionary: atomic number

n
Z

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

 
Measures and Units: atomic number

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.)

 

Number of a chemical element in the systematic, ordered sequence shown in the periodic table. The elements are arranged in order of increasing number of protons in the nucleus of the atom (the same as the number of electrons in the neutral atom), and that number for each element is its atomic number.

For more information on atomic number, visit Britannica.com.

 
Columbia Encyclopedia: 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.


 
Science Dictionary: atomic number

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.)

 
Wikipedia: atomic number

See also: List of elements by atomic number


In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is traditionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, the number of electrons also equals the atomic number.

The atomic number is closely related to the mass number, which is the number of protons and neutrons in the nucleus of an atom. The mass number defines the isotope of the element and often comes after the name of the element, e.g. carbon-14 (used in carbon dating).

History

The atomic number originally meant the number of an element's place in the periodic table. Dmitri Mendeleev arranged the known elements in increasing order of atomic weight and grouped by their similar chemical properties[1]. However, placing the elements in strict order of atomic weight resulted in some mismatches. Iodine and tellurium, if listed by atomic weight, appeared to be in the wrong order; and would fit better by chemical properties if their places in the table were swapped[2]. Placing them in the order which fit their properties most closely, their number in the table was their atomic number. This number appeared to be related to the mass of the atom but, as the discrepancy showed, reflected some property other than mass.

The anomalies in this sequence were finally explained after research by Henry Gwyn Jeffreys Moseley in 1913[3]. Moseley discovered a strict relationship between the x-ray diffraction spectra of elements, and their correct location in the periodic table. This led to the conclusion that the atomic number corresponds to the electric charge of the nucleus — the charge of the protons. The atomic number is the number of protons that is equal to the number of electrons.

Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of protons in its nucleus. The charge of an atom's nucleus defines its electron configuration based on principles of quantum mechanics. The form of each element's electron shells, particularly the valence shell, is the primary factor in determining its chemical bonding behavior.

New elements

The quest for new elements is usually described using atomic number. As of early 2007, elements with atomic numbers through 118 (excluding 117) have been discovered. Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created. In general, the half-life becomes shorter as atomic number increases, though an "island of stability" may exist for undiscovered isotopes with certain numbers of protons and neutrons.

See also

Notes


 
 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.  Read more
Measures and Units. A Dictionary of Weights, Measures, and Units. Copyright © Donald Fenna 2002, 2004. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
Science Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Atomic number" Read more

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