Atomic number is defined as which of the following?

Atomic number is defined as which of the following?

• A. The total number of neutrons in the nucleus
• B. The number of protons in the nucleus of an atom
• C. The total number of electrons surrounding the nucleus
• D. The average mass of all isotopes of an element

Answer: (B)

Atomic number is the number of protons in the nucleus. This count uniquely determines an element’s identity, since each element has a specific proton count. In a neutral atom, the number of electrons equals the number of protons, balancing the positive charge. The mass of an atom comes from protons and neutrons, so neutrons don’t set the atomic number, and the average mass of isotopes depends on how many neutrons vary among the isotopes. So the defining measure is the count of protons in the nucleus.

Atomic number is the number of protons in the nucleus. This count uniquely determines an element’s identity, since each element has a specific proton count. In a neutral atom, the number of electrons equals the number of protons, balancing the positive charge. The mass of an atom comes from protons and neutrons, so neutrons don’t set the atomic number, and the average mass of isotopes depends on how many neutrons vary among the isotopes. So the defining measure is the count of protons in the nucleus.