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

Number of protons in helium nucleus = 2

We know that,
Atomic mass = Number of protons + Number of neutrons

So, Number of neutrons = Atomic mass - Number of protons

Number of neutrons = 4 – 2 = 2

Hence, helium has 2 neutrons.