Question

Give reason for the following:

The mass number of an atom is slightly less than the actual atomic mass.

Answer 1

Mass number = number of protons + number of neutrons in an atom.
Now, mass of protons and neutrons is measured in daltons or atomic mass unit,u but the mass of each neutron and proton is slightly greater than 1 dalton, so the mass number ofan atom is slightly less than the actual atomic mass.
Example: mass umber of calcium = 20 + 20 = 40 u
Actual atomic mass of calcium = 40.078 u