Why Carbon-12 (C-12) atom is used …

The fact that the elements all seem to weigh multiples of the mass of hydrogen was noticed early in the history of chemical atoms, as soon as chemists started to determine atomic weight. They thought that all matter was made of hydrogen. But the element hydrogen was never used as an atomic mass standard, because it doesn't form compounds with enough other elements. The atom that does form compounds with every element known at that time, and almost all elements known now, is oxygen; and so for at least a century chemists assigned natural oxygen a mass of exactly 16 as a way of determining atomic weights by chemical methods.

Around the turn of the 20th Century, the physicists began measuring atomic weights of individual atoms, using mass spectrometry; they, too, used the oxygen standard, but they assigned the isotope O-16 a mass of exactly 16.

These standards are incompatible, because natural oxygen is a mixture of O-16, O-17 and O-18. For chemists, setting oxygen's average atomic mass to 16 was convenient, because chemists were determining the masses using bulk chemistry, but for mass spectrometrists it certainly was not a convenient standard!

The dilemma was solved by assigning C-12 a mass of exactly 12.

Because mass of one mole C₁₂  carbon is 12g