Final answer:
An element's unique chemical properties are primarily determined by its atomic number, the number of protons in its nucleus. The atomic number dictates the number of electrons, which intimates the element's chemical behavior. Isotopes have differing neutrons but do not change an element's chemical properties.
Step-by-step explanation:
The chemical properties of an element depend on the number of protons and electrons inside the atom. Each element is defined by its atomic number (Z), which is the unique number of protons in the nucleus of its atoms. Since protons are positively charged, in a neutral atom, the number of electrons, which are negatively charged, will equal the number of protons, ensuring a balance of charges.
Isotopes of an element have the same atomic number but differ in the number of neutrons. However, isotopes do not affect the chemical properties of an element because these properties are determined primarily by the arrangement of electrons, which in turn is influenced by the number of protons. Therefore, it's the atomic number that is key to an element's chemical identity, which answers why each element is unique and different from other elements in chemical properties. The correct choice from the given options is C) Each element has a unique number of protons in its nucleus.