The atomic number of an element is defined as the number of protons present in the nucleus of its atoms, while the atomic mass refers to the combined total of protons and neutrons in the nucleus.

Each element has a unique atomic number that determines its placement on the periodic table. This number corresponds directly to the number of protons in an atom’s nucleus. For instance, carbon has an atomic number of $6$, indicating that it contains $6$ protons.

In contrast, atomic mass is the total count of both protons and neutrons found within an atom’s nucleus. This measurement is expressed in atomic mass units (amu) and is typically presented as a decimal value on the periodic table. For example, carbon has an atomic mass of approximately $12 \, \text{amu}$, which is the sum of its $6$ protons and $6$ neutrons.

Understanding the distinction between atomic number and atomic mass is crucial, particularly when it comes to identifying isotopes of an element. Isotopes are variants of the same element that possess differing numbers of neutrons, leading to variations in their atomic masses. For example, carbon has isotopes known as carbon-12 and carbon-14. Carbon-12 has an atomic mass of $12 \, \text{amu}$ (composed of $6$ protons and $6$ neutrons), whereas carbon-14 has an atomic mass of $14 \, \text{amu}$ (comprising $6$ protons and $8$ neutrons).

In summary, both atomic number and atomic mass are fundamental properties of an atom, though they represent different facets of its structure. The atomic number indicates the number of protons, while the atomic mass signifies the total number of protons and neutrons.