The number of neutrons is not considered as the atomic number because the atomic number specifically refers to the number of protons in an atom's nucleus. The atomic number, denoted by the symbol Z, is a fundamental property of an element and determines its unique identity.
The proton number or atomic number represents the defining characteristic of an element because it determines the element's chemical properties. The number of protons defines the element's place in the periodic table and determines its atomic symbol. For example, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 because it has six protons.
Neutrons, on the other hand, are electrically neutral particles found in the nucleus of an atom. They contribute to the mass of the atom but do not significantly influence its chemical behavior. Isotopes of an element have the same atomic number (same number of protons) but different numbers of neutrons. The variation in neutron number among isotopes affects the atomic mass, but it does not alter the fundamental chemical properties of the element.
The distinction between the atomic number (proton number) and the number of neutrons is crucial in defining elements and understanding their properties. It allows for systematic organization in the periodic table and provides a basis for understanding chemical reactions and bonding behavior.