The number of electrons is not considered the atomic number because the atomic number specifically refers to the number of protons in an atom's nucleus.
The atomic number is a fundamental property of an element and determines its unique identity. Each element on the periodic table has a distinct atomic number, which corresponds to the number of protons in the nucleus of its atoms. For example, hydrogen has an atomic number of 1, meaning its atoms have one proton in the nucleus. Carbon has an atomic number of 6 because it has six protons in its nucleus.
While the number of electrons in an atom is usually equal to the number of protons to maintain overall electrical neutrality, the electrons themselves do not determine the identity of an element. Elements can exist in different ionization states where they gain or lose electrons, but they still retain the same atomic number.
The arrangement and distribution of electrons in an atom's electron shells and orbitals determine its chemical properties and behavior. These electrons participate in chemical bonding and determine an element's reactivity. However, the atomic number specifically refers to the number of protons and is used to classify elements in the periodic table and differentiate one element from another.