atomic number

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atomic number,

often represented by the symbol Z, the number of protonsproton,
elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
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 in the nucleus of an atomatom
[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom
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, as well as the number of electronselectron,
elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter.
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 in the neutral atom. Atoms with the same atomic number make up a chemical elementelement,
in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved.
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. 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 tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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 in the order of their atomic numbers. Mendeleev's periodic lawperiodic law,
statement of a periodic recurrence of chemical and physical properties of the elements when the elements are arranged in order of increasing atomic number.
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 was originally based on atomic weightsatomic weight,
mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent.
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. See mass numbermass number,
often represented by the symbol A, the total number of nucleons (neutrons and protons) in the nucleus of an atom. All atoms of a chemical element have the same atomic number (number of protons in the nucleus) but may have different mass numbers (from having
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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

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.

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.

atomic number

[ə′täm·ik ′nəm·bər]
(nuclear physics)
The number of protons in an atomic nucleus. Also known as proton number.

atomic number

the number of protons in the nucleus of an atom of an element.