Explain the mole concept.

Atoms and molecules are extremely small in size,and their numbers in even a small amount of any substance are very large.
In the $SI$ system,the mole (symbol: $mol$) was introduced as the seventh base quantity for the amount of a substance.
One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly $12 \ g$ (or $0.012 \ kg$) of the ${}^{12}C$ isotope.
To determine this number precisely,the mass of a carbon-$12$ atom was determined by a mass spectrometer and found to be $1.992648 \times 10^{-23} \ g$.
Knowing that one mole of carbon weighs $12 \ g$,the number of atoms in it is calculated as:
$\frac{12 \ g/mol \ {}^{12}C}{1.992648 \times 10^{-23} \ g/{}^{12}C \text{ atom}} = 6.0221367 \times 10^{23} \text{ atoms}/mol$.
This number of entities in $1 \ mol$ is known as the 'Avogadro constant',denoted by $N_{A}$,in honor of Amedeo Avogadro. Therefore,$1 \ mol$ of hydrogen atoms $= 6.022 \times 10^{23}$ atoms.