The atomic number of an atom tells you the number of protons in its nucleus.

Here's why this is important:

* Identity: The number of protons defines what element an atom is. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Electrons: In a neutral atom, the number of protons is equal to the number of electrons. This is crucial for understanding the atom's chemical behavior, as electrons are involved in bonding.

Important Note: The atomic number does not tell you the number of neutrons in an atom. The number of neutrons can vary, leading to isotopes of the same element.