The part of the atom that determines what element it is, is the number of protons in the atom's nucleus. This is known as the atomic number, which is unique to each element. For example, all atoms of carbon have six protons in their nucleus, which gives carbon its atomic number of 6. The number of protons also determines the chemical properties of the element and how it will interact with other elements in chemical reactions. Therefore, the number of protons is a fundamental characteristic of an atom that identifies it as a particular element.