Which of the following determines an atom's identity?

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

Answer: (B)

Verified step by step guidance

1. Understand that an atom's identity is defined by its atomic number, which corresponds to the number of protons in its nucleus.
2. Recognize that the number of neutrons can vary in atoms of the same element, leading to different isotopes, but this does not change the element's identity.
3. Note that the number of electrons can change when atoms become ions, but this also does not alter the element itself.
4. Realize that the total mass of the atom depends on both protons and neutrons, but mass alone does not determine the element's identity.
5. Conclude that the defining characteristic of an element is the number of protons in its nucleus.