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The atomic mass unit is also known as the unified mass unit or the dalton. It is a standard non-SI unit of mass. It is used to quantify mass on the atomic scale, for example, the mass of a proton, the mass of an electron, the mass of iron.
The number of nucleons present in an atom is called mass number. Mass number, as from the name, is responsible for the mass of an atom or atomic mass. Since protons and neutrons are much heavier than electrons, the mass of an atom is estimated by the number of protons and the number of neutrons or in other words, mass number. Mass number is always a whole number because the number of protons and neutrons present in an atom is always a whole number.
The Gold Book of the International Union of Pure and Applied Chemistry (IUPAC) defines atomic mass as “rest mass of an atom in its ground state”. In simple words, atomic mass is the mass of an atom of an element.
Today, we know every matter around us is composed of atoms. But this fact was a mystery until the end of the 18th century when Dalton, an English chemist, proposed his atomic theory. Dalton’s atomic theory was the first scientific atomic theory based on his experiments and examinations of previous scientific works. Modern atomic theory is much different from what Dalton had proposed, but some of the ideas of the theory are still valid. Dalton’s theory provided a foundation for modern chemistry.
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