The atomic number and the sum of protons and neutrons, also known as the mass number, are two different properties of an atom:
Atomic number (Z): The atomic number of an element represents the number of protons in the nucleus of an atom. It is a fundamental characteristic of an element and determines its identity. Each element on the periodic table has a unique atomic number, which is denoted by the symbol "Z". For example, the atomic number of carbon is 6, indicating that carbon atoms have 6 protons in their nuclei.
Mass number (A): The mass number of an atom represents the total number of protons and neutrons present in the nucleus. It is denoted by the symbol "A". Since neutrons and protons have similar masses, the mass number is roughly equal to the sum of protons and neutrons. However, in some cases, isotopes of an element may have different numbers of neutrons, resulting in different mass numbers. Isotopes are atoms of the same element with different numbers of neutrons but the same number of protons. For example, carbon-12 (C-12) and carbon-14 (C-14) are isotopes of carbon, with mass numbers of 12 and 14, respectively.
In summary, the atomic number (Z) specifically refers to the number of protons in an atom's nucleus and determines the element's identity. On the other hand, the mass number (A) represents the total number of protons and neutrons in the nucleus and can vary among different isotopes of an element.