The mass number and atomic number are both fundamental properties used to describe atoms, but they represent different aspects of an atom's identity.
Atomic Number: The atomic number (Z) of an atom represents the number of protons found in the nucleus of that atom. It is a unique identifier for each element on the periodic table. In a neutral atom, the atomic number also corresponds to the number of electrons surrounding the nucleus. For example, hydrogen has an atomic number of 1, indicating it has one proton and one electron, while carbon has an atomic number of 6, indicating it has six protons and six electrons.
Mass Number: The mass number (A) of an atom represents the total number of protons and neutrons present in the atom's nucleus. Unlike the atomic number, the mass number is not typically listed on the periodic table because it can vary for different isotopes of the same element. Isotopes are atoms of the same element that have different numbers of neutrons. The mass number is determined by summing the number of protons and neutrons in the nucleus. For example, carbon-12 has a mass number of 12 because it has six protons and six neutrons, while carbon-14 has a mass number of 14 because it has six protons and eight neutrons.
In summary, the atomic number represents the number of protons (and electrons) in an atom, providing information about the element's identity, while the mass number represents the total number of protons and neutrons, giving insight into the atom's mass and isotopic composition.