Difference between Atomic Number and Mass Number

the Major Difference between Atomic Number and Mass Number is that the 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 an 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 its identification number.

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

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

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

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

Element  Number of Protons Number of Neutrons Atomic Number Z Mass Number A
Hydrogen 1 0 1 1
Carbon 6 6 6 12
Nitrogen 7 7 7 14
Oxygen 8 8 8 16
Fluorine 9 10 9 19
Sodium 11 12 11 23
Magnesium 12 12 12 24
Potassium 19 20 19 39
Calcium 20 20 20 40


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 the 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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