Question

What does $C - 12$ in amu?

Answer 1

Hint: Atomic mass unit is the mass representing unit for the elemental atoms. It is described as the twelfth part of the mass of the carbon atom in which six neutrons and six protons are present. Hence the mass of an atom is equal to the mass of the nucleus.

Complete step by step answer:
Earlier the mass measurement of quantum particles such as electrons, protons and neutrons was a very difficult task. It was difficult to measure of atomic particles because they were very small and also not visible by naked eyes of human beings so to find them and weigh was very difficult task or we can say impossible task, but after a few decades the equipments were invented to see the atoms and atomic particles but there was still a problem related to mass of atoms. To tackle this problem a new unit was invented: an atomic mass unit or we can say amu. The atomic mass weighted of the elements was the average of mass, of all the isotopes of element. One atomic mass unit (amu) was $\frac{1}{{12}}th$ part of mass of carbon atom or $C - 12$ atom. This was done so that the mass calculation for elements can become easy. In carbon atoms six protons and six neutrons exist which sums up to twelve and hence carbon atom is represented as $C - 12$. So from here, using this concept the mass of atoms is calculated as the sum of number of protons and number of neutrons.
The formula used to calculate mass of an atom is:
${\text{Mass of atom = No}}{\text{. of protons + No}}{\text{. of neutrons}}$
In this formula now we have to calculate the number of protons, so the number of protons is nothing but the atomic number of an atom. Hence we can say that the atomic number and the mass number of an element are interrelated with each other.
So overall we can say that $C - 12$ is nothing but the atomic mass representation of carbon atoms in amu where amu is atomic mass unit or the average of proton rest mass and neutron rest mass.

Note:
The weighing of atoms or mass of atoms was decided by scientists on the basis of $C - 12$ atom because it has an exact whole number, no other element has whole number masses, their masses were in decimals due to existence of isotopes.