What characteristic of the atomic number ensures that every element is unique?

The atomic number, defined as the number of protons in the nucleus of an atom, is what makes each element unique. This number is fundamental because it determines the identity of the element. All atoms of a given element have the same atomic number, which means they contain the same number of protons. Since the number of protons is what defines the element, this characteristic ensures that each element is distinct from others.

For example, hydrogen has an atomic number of 1 because it has one proton, while helium has an atomic number of 2 due to having two protons. No atom can be classified as hydrogen if it has a different number of protons. This constancy of the atomic number across isotopes (which are atoms of the same element with different numbers of neutrons) further cements the idea that the atomic number is the defining trait of an element, thus ensuring their uniqueness.

The other choices do not accurately reflect the defining aspect of atomic numbers. Some indicate associations with mass or behavior during electron loss, while others suggest that different isotopes of the same element would have varying atomic numbers, which is incorrect. Isotopes remain the same element with the same atomic number despite their differing neutron counts.