In nuclear chemistry, what name is given to atoms of different elements that have the same mass number but different atomic numbers?

Introduction / Context:
In chemistry and nuclear physics, it is important to distinguish between different classifications of atoms based on their numbers of protons, neutrons and mass numbers. Terms such as isotopes, isobars, isotones and nuclear isomers all refer to specific relationships between these quantities. This question focuses on the situation where the mass number is the same but the atomic number differs, and tests whether you can recall the correct term used for such atoms.

Given Data / Assumptions:

• Mass number is the total number of protons and neutrons in the nucleus.
• Atomic number is the number of protons in the nucleus.
• Different elements have different atomic numbers.
• The question describes atoms that share the same mass number but have different atomic numbers.

Concept / Approach:
We begin by recalling the definitions. Isotopes are atoms of the same element with the same atomic number but different mass numbers. Isobars are atoms of different elements with the same mass number but different atomic numbers. Isotones are atoms that have the same number of neutrons but different atomic numbers and mass numbers. Nuclear isomers are atoms with the same atomic number and mass number but different energy states. The description in the question clearly matches the definition of isobars, so we should choose that term from the options.

Step-by-Step Solution:
Step 1: Note that the question specifically says same mass number but different atomic numbers. Step 2: Recall that mass number equals protons plus neutrons. Step 3: Write down the definition of isotopes and confirm they have same atomic number, which does not match the question. Step 4: Write down the definition of isobars, which states same mass number but different atomic numbers. Step 5: Match this definition to the wording in the question and select isobars as the correct term.

Verification / Alternative check:
A good way to verify is to use examples. Calcium 40 and argon 40 both have mass number 40 but different atomic numbers, so they are isobars. On the other hand, carbon 12 and carbon 14 are isotopes since they have the same atomic number but different mass numbers. Checking these known examples against the definitions confirms that the correct classification for atoms with equal mass number and different atomic number is isobars.

Why Other Options Are Wrong:
Isotopes have identical atomic numbers, which conflicts with the condition that atomic numbers are different.
Isotones have equal numbers of neutrons rather than equal mass numbers, so that definition does not fit here.
Isomers refer to nuclei with the same atomic and mass numbers but different energy states, which again does not match the question.
Allotropes relate to different physical forms of the same element in the same physical state, for example different crystalline forms of carbon, and are not defined by mass and atomic numbers of individual atoms.

Common Pitfalls:
Many learners confuse the terms isotopes and isobars because both include the syllable iso. A useful memory aid is that isobars share the same "bar" or total mass number, while isotopes share the same atomic number. Another common error is to ignore that the question explicitly mentions different atomic numbers and to select isotopes by habit. Reading the condition carefully will prevent this mistake.

Final Answer:
Atoms of different elements that have the same mass number but different atomic numbers are called isobars.