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All samples of an element are composed of atoms having the same number of protons. The number of protons determines its place on the periodic table and its atomic number. Therefore the atomic number is the amount of protons an element has contained within its nucleus. Elements can have various isotopes that each have a different mass. The answer is 1. atomic mass. 
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Answer is: 2. atomic number.

Atomic number is unique and defines an element. Atomic number (Z) is total number of protons in an atom.

For example, nitrogen atom (N-14) has 7 protons (p⁺), 7 electrons (e⁻) and 7 neutrons (n°). Protons (positive charge) and neutrons are in the nucleus of atom, electrons (negative charge) are bound to the nucleus in spherical shells.  Nitrogen is an element with atomic number 7.                                                       Mass number (A) is the total number of protons and neutrons in a nucleus. Nitrogen mass number is 14 (A = p⁺ + n°; A = 14).


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