Which of the following correctly describes the conditions under which two atoms are considered different isotopes of the same element?

• A. They have the same number of protons but different numbers of neutrons.
• B. They have different numbers of protons but the same number of neutrons.
• C. They have the same mass number but different atomic numbers.
• D. They have the same number of electrons but different numbers of protons.

Answer: (A)

Verified step by step guidance

1. Recall that isotopes are variants of the same chemical element, meaning they must have the same number of protons because the number of protons defines the element's identity (its atomic number).
2. Understand that while isotopes have the same number of protons, they differ in the number of neutrons, which affects their mass but not their chemical properties significantly.
3. Analyze the options given: the correct description of isotopes should state that the atoms have the same number of protons but different numbers of neutrons.
4. Recognize that options mentioning different numbers of protons describe different elements, not isotopes, and options mentioning the same mass number but different atomic numbers are contradictory because mass number depends on protons plus neutrons.
5. Conclude that the defining condition for isotopes is having the same atomic number (protons) but different neutron counts, which leads to different mass numbers.