What defines the atomic number of an element?

The atomic number of an element is defined by the number of protons in the nucleus of its atoms. This fundamental property is what distinguishes one element from another on the periodic table. Each element has a unique atomic number, which determines its identity and its position on the periodic table. For example, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 due to its six protons.

The atomic number is crucial because it not only identifies the element but also plays a key role in determining the chemical behavior of the element, including its electron configuration and reactivity. Therefore, it is this count of protons that signifies the atomic number, and as a result, it is foundational to the understanding of chemical elements in the field of chemistry.