The mass number and atomic number are two different properties that help characterize atoms. Here's the difference between the two:
Atomic Number: The atomic number of an atom represents the number of protons in the nucleus of that atom. It is denoted by the symbol "Z" and is a fundamental property of an element. The atomic number determines the identity of an element because each element has a unique number of protons. For example, all hydrogen atoms have an atomic number of 1, carbon atoms have an atomic number of 6, and so on. In a neutral atom, the atomic number also corresponds to the number of electrons surrounding the nucleus.
Mass Number: The mass number of an atom represents the total number of protons and neutrons in the nucleus of that atom. It is denoted by the symbol "A." Unlike the atomic number, the mass number can vary among atoms of the same element because different isotopes of an element have different numbers of neutrons. Isotopes are atoms of the same element with the same atomic number but different mass numbers. For example, carbon-12 (C-12) has a mass number of 12 because it has 6 protons and 6 neutrons, while carbon-14 (C-14) has a mass number of 14 because it has 6 protons and 8 neutrons.
To summarize, the atomic number represents the number of protons in an atom and determines the element's identity, while the mass number represents the total number of protons and neutrons in the nucleus and can vary among isotopes of the same element.