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A description of the nucleus:
 Symbols for Nuclear Quantities Symbol Name Explanation A Mass Number the number of nucleons (protons and neutrons) in the nucleus Z Atomic Number the number of protons in the nucleus N neutron Number The number of neutrons in the nucleus

An Example:

 Mass Number(A) Chemical Symbol 27 Al 13 Atomic Number (Z)

Here the symbol is for aluminum (aluminium for the rest of the world). A typical aluminum atom has a mass number  of 27 and an atomic number of 13. Therefore it has 13 protons(atomic number) and 14 neutrons
(27 - 13 = 14).

So the relationship is:

A = Z + N

In fact this is how the neutron number is calculated.  But what about atoms that have the same number of protons and different numbers of neutrons? The number of protons determines what the atom is, and some atoms have different numbers of neutrons. These  called isotopes.  For example, hydrogen has three isotopes:

 1 2 3 H (Hydrogen) H (Deuterium) H (Tritium) 1 1 1

Density of the Nucleus:

The nucleus is about 2.3 x 1014 times as dense as water. So a cubic meter of nuclear material would have a mass of 2.3 x 1017 kg.

Physicists today speak of the mass of the nucleus in terms of its rest energy. Yes that is correct. Mass is energy!  This concept is best demonstrated by Einstein’s special theory of relativity.   Rest energy is equal to the mass times the square of the speed of light.
Or E0= m c2.  The Energy is expressed in MeV or million electron volts, this makes the mass units MeV/c2, strange indeed.

 Mass of proton, neutron and electron Particle kg u Mev/c2 proton 1.673 x 10-27 1.007 276 938.3 neutron 1.675 x 10-27 1.008 665 939.6 electron 9.109 x 10-31 0.000 549 0.5110

 [The Nucleus and Beyond] [Properties of the Nucleus]