Final answer:
The identity of an element is determined by the number of protons in an atom, which is signified by the atomic number and corresponds to the unique placement of the element in the periodic table.
Step-by-step explanation:
The atomic number defines the identity of an element, as it describes the number of protons in the nucleus of an atom. Every atom has a unique number of protons that corresponds to its place in the periodic table, which is a defining trait of the element. For instance, all carbon atoms contain six protons. The number of protons also determines the number of electrons in a neutral atom, since the overall charge must be balanced. While the number of neutrons can vary in isotopes of an element, the number of protons is always constant, making it the key factor in determining the element's identity.