Question

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

Answer 1

No. of Protons + No. of Neutrons = Mass number.
The mass number of He = 4 u
Atomic number of He = 2 = Number of protons = Number of electrons

Using the formula,
No. of Neutrons = Mass number - No.of Protons
No. of Neutrons = 4-2 = 2
Therefore, No.of Neutrons in Helium atom = 2