Question

If 'u' denotes 1 atomic mass unit. One atom of an element has mass exactly equal to '$A$u', where $A$ is the mass number of element. Find A.

• A. $A=1$
• B. $A=12$
• C. $A=16$
• D. $A$ can take up any integral value from 1 to 110

Answer 1

Answer: (B)

$A=12$

$1$ atomic mass unit (amu or u) is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom.

$1u=\frac{1}{12}$ of C-12 atom

So, in Au, if $A=12$ then, that mass is exactly equal to one carbon-12 atom.

Therefore A = 12