Atomic number How Important It Is To A Chemist ?

In chemistry 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 Zahl 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 Atomzahl (and its English equivalent atomic number) come into common use in this context.

IMPORTANT

 Normal atoms have the same number of electrons as protons. The number of electrons is what makes each element behave a certain way in chemical reactions. So the atomic number, which is the number of protons and thus of electrons, is what makes oneelement different from another.
Hydrogen atoms have 1 proton, and thus an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!

Atoms of the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms "isotopes" of carbon.

Chemist also talk about the "atomic mass" of an atom. 

The nucleus of an atom contains nearly all (more than 99%) of an atom's mass. 

Neutrons and protons have almost exactly the same mass. So, to calculate atomic mass, we just add up the number of protons plus the number of neutrons. 

A carbon atom with 6 protons and 8 neutrons has an atomic mass of 14 ( = 6 + 8). Sometimes scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

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