In nuclear chemistry, isobars are defined as nuclides of different elements that have which of the following characteristics?

Introduction / Context:
In the study of atomic nuclei, it is useful to classify nuclides based on the relationships between their numbers of protons and neutrons. Three related concepts are isotopes, isobars, and isotones. This question focuses on isobars and asks which combination of mass number and atomic number correctly describes their defining feature. Understanding these definitions helps in interpreting nuclear reaction equations and nuclear stability charts.

Given Data / Assumptions:

• Mass number (A) equals the total number of protons plus neutrons in the nucleus.
• Atomic number (Z) equals the number of protons in the nucleus and determines the element.
• Isobars are a specific category of nuclides discussed in nuclear chemistry.
• We assume standard textbook definitions of isotope families.

Concept / Approach:
Isotopes are atoms of the same element (same atomic number) that have different mass numbers because of different neutron counts. Isobars, in contrast, are nuclides that have the same mass number but different atomic numbers, meaning they belong to different elements but have the same total number of nucleons. Isotones have the same number of neutrons but different atomic numbers. Since the question is about isobars, we must choose the description that explicitly mentions same mass number and different atomic number.

Step-by-Step Solution:
Step 1: Recall the definitions: isotopes share atomic number and differ in mass number, isobars share mass number and differ in atomic number, and isotones share neutron count. Step 2: Focus on isobars and identify that their hallmark is having the same mass number A. Step 3: Note that if the atomic numbers were also the same, they would be the same element, which is the case for isotopes, not isobars. Step 4: Therefore, isobars must have different atomic numbers while keeping mass number the same. Step 5: Choose the option stating that isobars have same mass numbers but different atomic numbers.

Verification / Alternative check:
Consider classic examples such as calcium 40 and argon 40. Calcium has atomic number 20, argon has atomic number 18, but both nuclides have mass number 40. They are different elements with the same total number of nucleons, so they are isobars. Another example is cobalt 60 and nickel 60. These pairs confirm that isobars share mass number but differ in atomic number. This is consistent with the definition selected and rules out descriptions that suggest equal atomic numbers or equal neutron counts.

Why Other Options Are Wrong:
- Different mass numbers but same atomic numbers: This describes isotopes, not isobars.
- Same mass numbers and same atomic numbers: This would mean the same nuclide, so it is not a distinct category at all.
- Different mass numbers and different atomic numbers: This is the general case for unrelated nuclides, not the specific relation called isobaric.
- Same number of neutrons but different mass numbers: This describes isotones, which share neutron count but differ in both atomic and mass numbers.

Common Pitfalls:
Students often mix up the three related terms. A helpful memory aid is that iso means same, and the second part tells you what is equal: isotopes share type (element) so they share Z, isobars share A because bar is linked to weight or mass, and isotones share neutrons. Confusing these leads to wrong answers. Taking a moment to connect each term with atomic number, mass number, or neutron count prevents such errors.

Final Answer:
Isobars are nuclides that have Same mass numbers but different atomic numbers.