Why is the mass of a carbon atom greater than the total mass of its protons and electrons?
The atomic mass is also contributed to by
The identity of the element is defined by the number of nuclear protons, massive, positively charged, nuclear particles.
The nucleus can contain various numbers of neutrons, neutrally charged massive particles, which can act to bind the nucleus together. Differing numbers of neutrons give rise to the existence of isotopes. For
The existence of isotopes allows the chemist to interrogate molecular structure by means of such experiments as mass spectroscopy, and (very importantly) by NMR spectroscopy.