What is amu?

A proton and a neutron have nearly equal masses – $1.66 \times 10^{-27} kg$ or $1.66 \times 10^{-24} gm$. Compare this with an electron which has mass of about $9.31 \times 10^{-31} kg$ or $9.31 \times 10^{-28} gm$, nearly 1837 times less than the mass of a proton. Therefore, while estimating the mass of an atom, we may safely ignore masses of electrons and consider contributions from protons and neutron only. Of course this will not yield the most accurate estimate of mass of an atom but it will get us a good enough estimate to work with on large scale.

We thus assume that mass of an atom is, to a good approximation, almost equal to the mass of its nucleus. Because an atomic nucleus consists of nucleons - proton and neutron - the mass of any nucleus must be an integral multiple of $1.66 \times 10^{-27} kg$ or $1.66 \times 10^{-24} gm$. For instance, if a nucleus has 14 protons and 13 neutrons, its mass is mass $27 \times 1.66 \times 10^{-24} gm$. Therefore, $1.66 \times 10^{-27} kg$ or $1.66 \times 10^{-24} gm$ is a fundamental number in which we express mass of all atoms. This measure of mass is referred to as atomic mass unit.
$$1 \, amu = 1.66 \times 10^{-27} kg = 1.66 \times 10^{-24} gm$$ Thus, an atom that has 14 protons and 13 neutrons has mass $27 \times 1.66 \times 10^{-24} gm = 27 \, amu$. Following this approach, the mass of a hydrogen atom ($^{1}_{1}H$) can be estimated to be $1 \, amu$ or $1.66 \times 10^{-24} gm$ because its nucleus consists of only a proton.

Mass of a helium atom ($^{4}_{2}He$) is 4 amu because its nucleus is composed of two protons and two neutrons. Similarly, mass of an oxygen atom ($^{16}_{8}O$, 8 protons and 8 neutron) is 16 amu; a nitrogen atom ($^{14}_{7}N$, 7 protons and 7 neutrons) has a mass of 14 amu. In general, an atom of an element $^{A}_{Z}X$ with $A$ nucleons has mass of $A \, amu$.

SUMMARY:
$1 \, amu = 1.66 \times 10^{-24} \, gm = \mbox{mass of one proton (or neutron)} = \mbox{mass of one hydrogen atom}$


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