The number of neutrons in a helium atom.  What’s given in the problem  The atomic mass of helium is 4 u. The number of protons in helium is 2.  Helpful information  Atomic mass is the sum of protons and neutrons.  How to solve  Subtract the number of protons from the atomic mass to find the number of neutrons.  The number of neutrons is the atomic mass minus the number of protons. Number of neutrons 4-2=2. Solution  The helium atom has 2 neutrons.