Question
What is the modern basis of atomic mass?

Answer

Nowadays the atomic mass is determined according to $C -12$ standard. According to this, the atomic number of an element is the number from which it is known that any atom is how much heavier than
$\frac{1}{12}$ part of $C -12$. Hence one atomic mass unit $\frac{1}{12}^{\text {th }}$
part of mass of $C -12$.
$
1 amu=1.66056 \times 10^{-24} gm
$

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