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

Atom:

• The atomic number($\mathrm{Z}$) of an atom is the number of protons present in that atom.
• For a neutral atom, the number of protons and neutrons will be the same.
• The mass number($\mathrm{A}$) of an atom is the sum of protons and neutrons present in that atom.

Helium:

• The given number of protons is 2, that is the atomic number of $\mathrm{He}$ atom is 2.
• The given mass number is $4\mathrm{u}$, which means the sum of protons and neutrons present in that atom.
• So, the number of neutrons is the difference between the mass number and the number of protons.
• The number of neutrons is $4-2=2$

Therefore, the number of neutrons is 2.