Which quantity determines the identity of an atom?

• A. The number of electrons
• B. The number of neutrons
• C. The atomic mass
• D. The number of protons

Answer: (D)

Verified step by step guidance

1. Understand that the identity of an atom is defined by its atomic number, which is the number of protons in the nucleus.
2. Recall that the number of protons determines the element because each element has a unique number of protons.
3. Note that the number of electrons can vary in ions, and the number of neutrons can vary in isotopes, but these do not change the element's identity.
4. Recognize that atomic mass is the sum of protons and neutrons, so it does not uniquely identify the element.
5. Conclude that the quantity that determines the identity of an atom is the number of protons.