The atomic number of an element is determined by the number of which particle in the nucleus?

The atomic number of an element is defined by the number of protons present in the nucleus of an atom. This is a fundamental property of an element that determines its identity; for example, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 due to having six protons.

The atomic number plays a crucial role in the periodic table, as elements are arranged in order of increasing atomic number. This also influences the chemical behavior and properties of the element, as the number of protons affects the number of electrons in a neutral atom, which in turn influences how the element interacts with others.

The other particles mentioned do not determine the atomic number: neutrons do not affect this measurement as they are neutral; electrons are involved in chemical bonding and charge neutrality but not in defining the atomic number; and the term "nuclei" refers to the plural form of nucleus or centers of atoms containing protons and neutrons, rather than a specific particle. Thus, the correct determination of atomic number points directly to the protons in the nucleus.