atomic number, often represented by the symbol Z, the number of protons proton, 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.
..... Click the link for more information. in the nucleus of an atom atom [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

..... Click the link for more information. , as well as the number of electrons electron, 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.
..... Click the link for more information. in the neutral atom. Atoms with the same atomic number make up a chemical element element, 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.
..... Click the link for more information. . 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 periodic 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
..... Click the link for more information. in the order of their atomic numbers. Mendeleev's periodic law periodic 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 .
..... Click the link for more information. was originally based on atomic weights. See mass number mass 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
..... Click the link for more information. .

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.

atomic number

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

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 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