In atomic structure, atoms of the same chemical element that have different mass numbers (because they differ in the number of neutrons) are called what?

Introduction / Context:
Atomic structure terminology includes several similar sounding words: isotopes, isobars, isotones, and isomers. These terms describe relationships between atoms or nuclei based on atomic number, mass number, and neutron count. Understanding the precise meaning of each is crucial in nuclear chemistry and basic physics. This question asks specifically about atoms of the same element with different mass numbers, a concept that appears in radioactivity, medicine, and dating methods.

Given Data / Assumptions:

• The atoms belong to the same element, meaning they have the same atomic number (same number of protons).
• They have different mass numbers, meaning the total number of protons plus neutrons differs.
• The difference in mass number arises because the number of neutrons is not the same.
• The options are isobars, isotopes, isotones, and isomers.

Concept / Approach:
Isotopes are defined as atoms of the same element that have the same atomic number but different mass numbers due to differing numbers of neutrons. Classic examples include hydrogen, deuterium, and tritium or carbon-12 and carbon-14. Isobars, in contrast, have the same mass number but different atomic numbers (different elements). Isotones share the same number of neutrons but have different atomic and mass numbers. Isomers are molecules or nuclear states with the same formula or nucleon count arranged differently. Given that the question clearly fixes the element (same atomic number) and varies the mass number, we are dealing with isotopes.

Step-by-Step Solution:
Step 1: Identify the constant quantity. The atoms are of the same element, so atomic number (number of protons) is the same. Step 2: Identify the varying quantity. Mass number differs, meaning the total number of protons plus neutrons is not the same. Step 3: Recall the formal definition. Isotopes: same atomic number, different mass numbers. Step 4: Match the definition with the options and select isotopes.

Verification / Alternative check:
Consider the isotopes of carbon: carbon-12 and carbon-14. Both have atomic number 6 (6 protons) but mass numbers 12 and 14, corresponding to 6 and 8 neutrons respectively. Textbooks refer to these as isotopes of carbon. If we look at iron-56 and nickel-56, they share the same mass number 56 but not the same atomic number; these would be called isobars, not isotopes. This clear example shows that the question's description fits the isotope definition exactly, confirming that isotopes is the correct choice.

Why Other Options Are Wrong:
Option A (Isobars): Isobars have the same mass number but different atomic numbers, meaning they are different elements, not the same element. Option C (Isotones): Isotones have the same number of neutrons but differ in both atomic and mass numbers, again not describing atoms of the same element. Option D (Isomers): Chemical isomers have the same molecular formula but different arrangements of atoms; nuclear isomers have the same nucleon numbers but different energy states; neither matches the description given.

Common Pitfalls:
Students often confuse isotopes and isobars due to the similar sounding names. A useful memory aid is that iso means same and the second part of the word tells you what is the same: isotopes share the same "top" number of protons (atomic number), isobars share the same "bar" or mass number, and isotones share the same neutrons. Keeping these definitions straight avoids confusion in nuclear chemistry questions.

Final Answer:
Atoms of the same element with different mass numbers are called Isotopes.