The total mass of an atom can be less than the combined mass of its individual protons and neutrons due to a concept in physics known as "mass defect" or "nuclear binding energy."
In an atomic nucleus, protons and neutrons are bound together by the strong nuclear force, which is a very powerful force that holds these particles together. However, this binding process requires energy, and as per Einstein's famous equation E=mc², energy and mass are interchangeable. Therefore, when nucleons (protons and neutrons) come together to form a nucleus, a tiny amount of mass is converted into binding energy to hold the nucleus together.
This mass defect is the difference between the total mass of an atom and the sum of the masses of its individual protons and neutrons. The mass defect is typically expressed in atomic mass units (amu) or MeV/c² (million electron volts per speed of light squared). One atomic mass unit is approximately equal to the mass of a proton or a neutron.
The binding energy is released when a nucleus is formed, and it helps to stabilize the nucleus. It's essentially the energy required to break apart the nucleus back into its individual nucleons. This energy is responsible for the tremendous power output of nuclear reactions, such as those that take place in nuclear power plants or nuclear explosions.
To summarize, the total mass of an atom is less than the sum of its individual nucleons (protons and neutrons) due to the conversion of a tiny amount of mass into nuclear binding energy, which is a manifestation of Einstein's mass-energy equivalence.