What is the number of protons called?

• A. Atomic mass
• B. Atomic number
• C. Isotope
• D. Valence

Answer: (B)

The number of protons in an atom’s nucleus is the atomic number, and it identifies the element. Each element has a unique atomic number, which is why this value appears on the periodic table as its defining property. The mass number, by contrast, is the total count of protons and neutrons, so it changes with different isotopes of the same element. Isotopes are versions of an element that share the same atomic number but have different numbers of neutrons, affecting mass but not the element’s identity. Valence refers to the outer-shell electrons involved in bonding, not to the proton count. So the term for the number of protons is atomic number.