Difference between Atomic Number and Mass Number of elements

What is the difference between the atomic mass and the atomic number?

Difference between Atomic Number and Mass Number:

Here we shall discuss the difference between atomic number and 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 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


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