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Atomic number and mass number

Atomic number (z): 
The atomic number of an element may be defined as the number of protons present in the nucleus of an atom or the number of electrons present in the extra nuclear part.It is denoted by z.
                 Atomic number,z=Number of protons in the nucleus
                                                =  Number of electrons in the extranuclear part
Mass number(a):
The discovery of neutrons made it clear that the nucleus of an atom consists of protons and neutrons. Since the electrons have negligible mass,the entire mass of the atom is mainly due to protons and neutrons present in the nucleus.Thus,the sum of the number of protons and neutrons present in the nucleus of an atom is called mass number.protons and neutrons are collectively called nucleons.
Mass number is denoted by 'a'.
                       Mass number = number of protons+ number of neutrons
                                                    = atomic number+number of neutrons
calculation of number of protons,electrons and neutrons,
knowing the mass number and the atomic number of an element,the number of protons,electrons and neutrons can be calculated.
For example,the mass number of the element sodium(na)is 23 and its atomic number is 11.
 Therefore,number of protons = atomic number=11
                     number of electrons=number of protons=11
                     number of neutrons= mass number-atomic number=23-11=12.
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