Isotopes of an element have

1. Analyze the question

The question asks for the defining characteristic of isotopes. To answer this, we must recall the atomic structure of elements and how isotopes differ from one another.

2. Explain the concept

• Atomic Number (Z): This is the number of protons in the nucleus of an atom. It defines the identity of an element (e.g., all Carbon atoms have 6 protons).
• Mass Number (A): This is the sum of protons and neutrons in the nucleus.
• Isotopes: These are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different number of neutrons).

Because the number of protons is the same, the number of electrons in a neutral atom of an isotope remains the same as well. However, the variation in the number of neutrons is the primary reason why isotopes have different masses.

3. Show the solution

• Step 1: Identify the constant. Isotopes belong to the same element, so they must have the same number of protons.
• Step 2: Identify the variable. Isotopes have different atomic masses, which is caused by a difference in the number of neutrons.
• Step 3: Evaluate the options: • Option A: States they have the same protons but different neutrons. This matches our definition perfectly. • Option B: Incorrect, because if the number of neutrons were the same, they would not be isotopes. • Option C: Incorrect, because if both protons and neutrons were the same, the atoms would be identical, not isotopes. • Option D: While isotopes do have the same number of protons and electrons, this statement describes the property of atoms of the same element in general, but it fails to explain why they are isotopes (which specifically requires the mention of the difference in neutrons).

4. Identify the answer

The correct option is A.

5. Exam Tip

Think of isotopes as "siblings" in the periodic table: they share the same "family name" (the element identity/proton count), but they have different "weights" (neutron count). Always look for the option that mentions the difference in neutrons to identify isotopes correctly!