Chemistry
Give reasons:
Mass number is slightly less than the actual atomic mass.
Atomic Structure
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Answer
Mass number is slightly less than the actual atomic mass because it is calculated by taking the mass of each proton and neutron as exactly 1 a.m.u. and by neglecting the mass of electrons.
However, the actual atomic mass is determined experimentally and includes the true masses of protons, neutrons and electrons. Since protons and neutrons are each slightly heavier than 1 a.m.u., and electrons also add a very small mass, the actual atomic mass is slightly greater than the mass number.
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