Question

Definition of atomic mass.
{\text{1 amu = }}\dfrac{{\text{1}}}{{{\text{12}}}}\;{\text{of a carbon atom}} \\\ \;{\text{ = }}\dfrac{{\text{1}}}{{{\text{12}}}}{{ \times }}\dfrac{{{\text{12}}}}{{{\text{6}}{{.023 \times 1}}{{\text{0}}^{{\text{23}}}}}}{\text{ = 1}}{{.66 \times 1}}{{\text{0}}^{{\text{ - 24}}}} \\\ \\\

Answer 1

Atoms are relatively small compounds. But as all matter has mass, the mass of an atom is also defined. Atomic mass is defined as the mass of an atom. It is the equal to the sum of mass of individual particles of an atom.

Complete step by step answer:
Atomic mass is the mass of an atom. We know that the SI unit of mass is kilogram. But atomic mass is expressed in atomic mass units(amu). One atomic mass unit is defined as the one by twelfth mass of a carbon atom. It can be shown mathematically as;

${\text{1 amu = }}\dfrac{{\text{1}}}{{{\text{12}}}}\;{\text{of a carbon atom}} \\\ \;{\text{ = }}\dfrac{{\text{1}}}{{{{12}}}}{\text{ \times }}\dfrac{{{\text{12}}}}{{{\text{6}}{{.023 \times 1}}{{\text{0}}^{{\text{23}}}}}}{\text{ = 1}}{{.66 \times 1}}{{\text{0}}^{{\text{ - 24}}}} \\\ \\\$
From this relation we can say that the mass of a carbon atom will be ${\text{12 amu}}$.
Atomic mass can also be calculated from the masses of the three main subatomic particles in an atom which are protons, neutrons and electrons. From the above relation we found out that the mass of a carbon atom is ${\text{12 amu}}$. We know that there are 6 protons and 6 neutrons in a carbon atom having atomic number 6. The mass of a proton and a neutron is approximately 1. So, when we take the sum of the masses of these protons and neutrons, we get the mass equal to ${\text{12 amu}}$.
When we defined the atomic mass unit, we came across a number, that is, ${\text{6}}{{.023 \times 1}}{{\text{0}}^{{\text{23}}}}$. This number is known as the Avogadro’s number. It is usually used to show the amount of a compound in a particular measure of it, say a mole. According to this fact, Avogadro’s number is the number of units which may be electrons, atoms, molecules etc. present in one mole of that substance.

Note: Avogadro’s number is essential to carry out calculations that include large numbers of compounds. We know that if one mole of any substance contains ${\text{6}}{{.023 \times 1}}{{\text{0}}^{{\text{23}}}}$ number of particles, we cannot always write this big number and instead we use the word mole.

Although mass can be expressed in terms of kilograms or grams, atoms being microscopic elements do not use that unit. Instead a special unit called the atomic mass unit is defined by scientists to make chemical calculations easier.