Here's why:

* Atomic Number: The atomic number of an element is the number of protons in the nucleus of an atom of that element.

* Protons: Protons are positively charged particles found in the nucleus of an atom. They determine the element's identity.

* Unique Identity: Each element has a unique number of protons. For example, all platinum atoms have 78 protons, while all gold atoms have 79 protons.

Therefore, platinum's atomic number of 78 defines its identity as platinum, distinguishing it from all other elements.