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WHAT IS DIFFERENT BETWEEN ATOMIC NUMBER AND MASS NUMBER

  The atomic number is the number of protons in an atom of an element. Atoms have no overall electrical charge. That means that there must be a balance between the positively charged protons and the negatively charged electrons. Atoms must have equal numbers of protons and electrons. 

For any element:
Number of Protons = Atomic Number
Number of Electrons = Number of Protons = Atomic Number

The sum of the total number of protons and neutrons present in the nucleus of an atom of an element is called mass number.For example ,
 Mass number of carbon is 12 because it has 6 protons and 6 neutrons (6 + 6 = 12) Similarly, mass number of aluminium is 27 (13 p + 14 n) .