Final answer:
The atomic number is the number of protons in an atom's nucleus, while the mass number is the sum of protons and neutrons.
Step-by-step explanation:
The atomic number (Z) is the number of protons in an atom's nucleus. It is a unique identifier for each element. For example, carbon has 6 protons, so its atomic number is 6.
The mass number (A) is the sum of protons and neutrons in an atom's nucleus. It is denoted by a superscript before the element symbol. For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12.