Atoms of elements possess unique properties that are essential for understanding chemistry. Three fundamental properties to be familiar with are the atomic number, mass number, and atomic mass (or atomic weight).

The atomic number is defined as the total number of protons in the nucleus of an atom. This number is crucial because it uniquely identifies an element; changing the number of protons alters the element itself. For example, a carbon atom has an atomic number of 6, indicating it contains 6 protons. If a seventh proton were added, the atom would become nitrogen, demonstrating that only the number of protons determines the element.

Next, the mass number represents the total mass of the nucleus of a single atom, calculated by adding the number of protons and neutrons present. For instance, if a carbon atom has 6 protons and 6 neutrons, its mass number would be 12, as calculated by the equation:

Mass Number = Number of Protons + Number of Neutrons

In this case, 6 (protons) + 6 (neutrons) = 12 (mass number).

Lastly, the atomic mass (or atomic weight) differs from the mass number in that it represents the average total mass of all the isotopes of an element, rather than the mass of a single atom. For carbon, the atomic mass is approximately 12.011, reflecting the average mass of its isotopes. This distinction is important as the atomic mass is not a whole number, unlike the mass number, which is always an integer.

In summary, the atomic number defines the element, the mass number provides the total count of protons and neutrons in a specific atom, and the atomic mass offers an average of all isotopes of that element. Understanding these properties lays the groundwork for further exploration of atomic structure and behavior in chemistry.