Wiktionary
n. (context physics chemistry English) The number, equal to the number of protons in an atom that determines its chemical properties. Symbol: Z
WordNet
n. the order of an element in Mendeleyev's table of the elements; equal to the number of protons in the nucleus or electrons in the neutral state of an atom of an element
Wikipedia
In chemistry and physics, the atomic number of a chemical element (also known as its proton number) is the number of protons found in the nucleus of an atom of that element, and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.
The atomic number, Z, should not be confused with the mass number, A, which is the number of nucleons, the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N (these quantities are always whole numbers). Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the " relative isotopic mass"), is roughly (to within 1%) equal to the whole number A.
Atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.
The conventional symbol Z comes from the German word meaning number/numeral/figure, which, prior to the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word (and its English equivalent atomic number) come into common use in this context.
Usage examples of "atomic number".
Element 101 has indeed been discovered since Campbell wrote Marooned but it is named mendelevium and it is unstable, as are all elements beyond atomic number 83.
More important, its atomic number was twenty-nine by theory elementary enough for any race that could build rockets.
Generally speaking, once you get past polonium at atomic number 84 everything is radioactive, it spits out particles of its own accord and 'decays' into something else, and the greater an element's atomic number, the more rapidly it decays.
The number of positively charged particles is what they call the 'atomic number,' and these particles, together with the uncharged particles, make up the 'atomic weight.
But the stuff that was turning up in the chemistry labs to replace the tungsten-186 and the osmium-186 had an atomic number of 94, no question about that.
I knew that the bomb would trigger an inextinguishable atomic fire affecting all elements with an atomic number above 10.
The chemistry of an atom depends only on the number of electrons, which equals the number of protons, and which is called the atomic number.
All I can say is that its atomic number is higher than any known now.
The warheads had the characteristic of generating an inextinguishable atomic holocaust which affected all elements having an atomic number higher than 10.