What does the atomic number of an element signify?

The atomic number of an element is a fundamental property that defines the identity of that element. It is equal to the number of protons present in the nucleus of an atom. Since the number of protons determines the element's position on the periodic table and its chemical properties, the atomic number is key to understanding what an element is and how it behaves chemically.

For instance, hydrogen has an atomic number of 1, which means it has one proton in its nucleus, whereas carbon has an atomic number of 6, indicating it has six protons. This distinction is crucial because elements with different atomic numbers are entirely different substances with different properties. Therefore, knowing the atomic number gives you direct insight into both the identity and the characteristics of the element.

Other choices do not correctly represent the significance of the atomic number. While it relates to aspects such as electron count in a neutral atom, density, or isotopes, these are secondary implications rather than a direct definition of the atomic number itself. The atomic number's primary role is to serve as a unique identifier for each element based on its proton count.