The mass of an atom is not smaller than the sum of its constituents. In fact, the mass of an atom is equal to the sum of the masses of its constituents, which include protons, neutrons, and electrons.
However, it is true that the mass of an atom is slightly smaller than the sum of its individual constituents when they are considered separately. This is due to a phenomenon called mass defect or mass deficiency. Mass defect arises from the conversion of mass into energy according to Einstein's mass-energy equivalence principle (E = mc²), where E is energy, m is mass, and c is the speed of light.
Inside the nucleus of an atom, protons and neutrons are bound together by the strong nuclear force. During the formation of a nucleus, some of the mass is converted into binding energy, which holds the nucleus together. This binding energy is released when nucleons (protons and neutrons) come together to form a nucleus.
The conversion of mass into binding energy reduces the total mass of the nucleus. Therefore, the mass of an atom is slightly less than the sum of the masses of its individual protons and neutrons. This mass defect is most noticeable for heavier elements with larger nuclei.