Helium atom has an atomic mass of 4 u
Question.
Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Solution:
The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Since helium atom has an atomic mass of 4u and it has two protons, the protons contribute 2u to atomic mass. Hence, it must contain 2 neutrons as each neutron contributes 1u to the nucleus.
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