Oxygen has an atomic number of 8 because the atomic number of an element corresponds to the number of protons in the nucleus of an atom of that element. The atomic number uniquely identifies each element on the periodic table.
In the case of oxygen, its atomic number of 8 indicates that there are eight protons in the nucleus of an oxygen atom. This is a fundamental property of oxygen that distinguishes it from other elements. The number of protons in an atom determines its chemical properties and its place in the periodic table.
Oxygen also has other associated properties, such as its atomic mass, electron configuration, and chemical behavior, which are determined by the arrangement of its protons, neutrons, and electrons. However, the atomic number specifically refers to the number of protons and serves as a fundamental characteristic of the element.