Atomic number is the ...

Atomic number is the ...

• A. The total number of electrons in an atom.
• B. The number of protons in the nucleus.
• C. The sum of protons and neutrons.
• D. The atomic mass.

Answer: (B)

The atomic number is the number of protons in the nucleus, and that count uniquely identifies an element on the periodic table. This fixed proton count defines the element’s identity, and in a neutral atom the number of electrons matches that same count, though atomic number itself is about protons, not electrons. The total electrons can vary with charge, the sum of protons and neutrons gives the atomic mass (which changes with isotopes), and the atomic mass is not the atomic number.

The atomic number is the number of protons in the nucleus, and that count uniquely identifies an element on the periodic table. This fixed proton count defines the element’s identity, and in a neutral atom the number of electrons matches that same count, though atomic number itself is about protons, not electrons. The total electrons can vary with charge, the sum of protons and neutrons gives the atomic mass (which changes with isotopes), and the atomic mass is not the atomic number.