Which of the following defines the atomic number of a given element?

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

Answer: (A)

Verified step by step guidance

1. Understand that the atomic number is a fundamental property of an element that uniquely identifies it.
2. Recall that the atomic number corresponds to the number of protons in the nucleus of an atom.
3. Recognize that electrons and neutrons do not define the atomic number; electrons equal protons in a neutral atom, and neutrons vary among isotopes.
4. Eliminate options that mention the total number of electrons and neutrons, the number of neutrons alone, or the sum of protons and electrons, as these do not define the atomic number.
5. Conclude that the correct definition of atomic number is the number of protons in the nucleus.