Answer:
C. The sum of the masses of protons and neutrons in an atom.
Step-by-step explanation:
We know that an atom consist of electrons, protons and neutrons. Neutrons and protons are present inside the nucleus while electrons are present out side the nucleus. Electron has a negative charge and is written as e⁻. The mass of electron is 9.10938356×10⁻³¹ Kg . While mass of proton and neutron is 1.672623×10⁻²⁷Kg and 1.674929×10⁻²⁷ Kg respectively.
Symbol of proton= P⁺
Symbol of neutron= n⁰
The number of proton or number of electrons are called atomic number while atomic mass of an atom is sum of mases of protons and neutrons. An electrically neutral atom always have equal number of protons and electrons and when an atom loses its valance electron the number of protons increases and thus positive charge increased and atom form cation.