2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Given, atomic mass of helium atom = 4 u and 2 protons in helium nucleus
We know that, atomic mass = number of protons + number of neutrons
So, 4 = 2 + number of neutrons
=> Number of neutrons = 4 – 2 = 2
Hence, Helium has 2 neutrons.