D. The atomic number of an element represents the number of protons in an atom of that specific element. It is a fundamental characteristic of an element and determines its position in the periodic table. The atomic number uniquely identifies each element and is denoted by the letter "Z." The sum of protons and neutrons in an atom is known as the mass number, and it can vary for different isotopes of an element. The sum of protons and electrons in a neutral atom is always equal to the atomic number.