Which quantity represents the number of protons in an atom?

Which quantity represents the number of protons in an atom?

• A. Mass number
• B. Atomic number
• C. Isotopes
• D. Number of neutrons

Answer: (B)

The number of protons in an atom is captured by its atomic number, which uniquely identifies the element. This quantity counts how many positive charges are in the nucleus, and in a neutral atom that same number also equals the electrons surrounding the nucleus. The mass number, by contrast, adds protons and neutrons together, so it changes if neutrons vary. Isotopes are versions of the same element that have the same number of protons but different numbers of neutrons, and the number of neutrons alone does not tell you how many protons there are. For example, carbon has six protons, so its atomic number is six, regardless of how many neutrons it contains.

The number of protons in an atom is captured by its atomic number, which uniquely identifies the element. This quantity counts how many positive charges are in the nucleus, and in a neutral atom that same number also equals the electrons surrounding the nucleus. The mass number, by contrast, adds protons and neutrons together, so it changes if neutrons vary. Isotopes are versions of the same element that have the same number of protons but different numbers of neutrons, and the number of neutrons alone does not tell you how many protons there are. For example, carbon has six protons, so its atomic number is six, regardless of how many neutrons it contains.