The atomic number defines the element. Which term describes this?

• A. Atomic Mass
• B. An Ion
• C. Atomic Number
• D. Isotope

Answer: (C)

The defining feature that identifies an element is the number of protons in its nucleus. This count is called the atomic number, and it is unique to each element on the periodic table. Because the atomic number fixes the element’s identity, the term that describes this feature is atomic number. Atomic mass reflects protons plus neutrons and can vary with isotopes, so it does not uniquely identify the element. An ion is an atom with a net charge, involving electrons, not the element’s identity. Isotopes are atoms of the same element with different numbers of neutrons, so they share the same atomic number but differ in mass.