Differentiate among atomic number, mass number, and isotopes, and which of these distinguishes one element from another

Atomic Number: The atomic number of an element is defined as the number of protons present in the nucleus of an atom of that element. It is unique for each element and is the fundamental property that defines the identity of an element. For example, the atomic number of hydrogen is 1, helium is 2, and so on. The atomic number is also equal to the number of electrons in an atom in its neutral state.

Mass Number: The mass number of an element is the total number of protons and neutrons present in the nucleus of an atom of that element. Unlike the atomic number, the mass number of an element can vary because the number of neutrons can vary. For example, the mass number of hydrogen can be 1 (1 proton + 0 neutron) or 2 (1 proton + 1 neutron).

Isotopes: Isotopes are atoms of the same element (i.e., they have the same atomic number) but have different mass numbers due to the different number of neutrons in their nuclei. For example, hydrogen has three isotopes: protium (1 proton + 0 neutron), deuterium (1 proton + 1 neutron), and tritium (1 proton + 2 neutrons).

Among these three, the atomic number is the one that distinguishes one element from another. As mentioned, each element has a unique atomic number, and it is this number that defines the identity of an element. The number of protons (which is the atomic number) in an atom determines the chemical properties of the element. Therefore, changing the number of protons changes the element itself.