An element and its isotope only differ in the number of

1. Analyze the question

The question asks for the fundamental subatomic difference between an element and its isotope. To answer this, we must recall the definitions of atomic structure and isotopes.

2. Explain the concept

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

3. Show the solution

• If an atom changes its number of protons, it becomes a completely different element (e.g., changing from Carbon to Nitrogen).
• If an atom changes its number of electrons, it becomes an ion (charged particle), not a different isotope.
• If an atom changes its number of neutrons, the chemical identity (protons) remains the same, but the mass of the atom changes. This is the definition of an isotope.

Therefore, isotopes are atoms of the same element that differ only in their neutron count.

4. Identify the answer

Comparing our explanation to the given options:

• A is incorrect because protons define the element.
• B is incorrect because electrons determine the charge/chemical bonding.
• C is incorrect because ions are charged atoms.
• D is correct because the variation in neutrons is what creates isotopes.

5. Tip

Memory Trick: Think of the word "Isotope" as having an "n" for "neutrons".

• Isotope = Same protons, different neutrons.