Difference Between Atomic Number and Mass Number

the Major Difference between Atomic Number and Mass Number is that mass number is the weight of the nucleus of an atom and is written on the upper left-hand corner of an elemental expression while the atomic number is the number of protons present in an atom and not affected by the number of neutrons.

Atomic Number Vs. Mass Number

The atomic number of an element is equal to the number of protons present in the nucleus of its atoms. ‘Z’ is the symbol of atomic number. As all atoms of an element have the same number of protons in their nuclei, they have the same atomic number. Hence each element has a specific atomic number termed as its identification number.


For example, all hydrogen atoms have 1 proton, their atomic number is Z=6. Similarly, in oxygen all atoms have 8 protons having atomic number Z=8 and sulphur having 16 protons shows atomic number Z=16.

While, the mass number is the sum of numbers of protons and neutrons present in the nucleus of an atom. it is represented by symbol’A’ .

It is calculated as A= Z+n ,     where n is the number of neutrons.

Each and neutron has 1 amu mass. For example hydrogen atom has one proton and no neutron in its nucleus, its mass number A=1+0 = 1. Carbon atom has 6 protons and 6 neutrons, hence its mass number A=12. Atomic numbers and mass numbers of a few elements are given in table bellow.

Element Number of ProtonsNumber of NeutronsAtomic Number ZMass Number A


How many protons and neutrons are there in an atom having A= 238 and Z=92.


First of all, develop data from the given statement of the example and then solve it with the help of data.


A=238      Z=92    Number of Protons= ?   Number of Neutrons  = ?

Number of protons=Z=92      Number of Neutrons=A-Z = 238-92= 146

Atomic number Mass number
It is the total number of protons present in the nucleus of an atom.It is the sum of total number of protons and neutrons in the nucleus of an atom.
It is represented by Z.It is represented by A.

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