In nuclear chemistry, isotones are nuclides that always have which feature in common?

Introduction / Context:
In nuclear chemistry and atomic physics, several related terms are used to compare different nuclides, including isotopes, isobars and isotones. Each term highlights a different way in which nuclei can be similar or different. Isotopes have the same number of protons, isobars have the same mass number and isotones have the same number of neutrons. This question checks whether you know the precise definition of isotones and can distinguish it clearly from the definitions of isotopes and isobars.

Given Data / Assumptions:

• We are comparing nuclides in terms of their numbers of protons, neutrons and electrons.
• Atomic number is equal to the number of protons in the nucleus.
• Mass number is equal to the total number of protons plus neutrons.
• The options describe different possible relationships between nuclides.

Concept / Approach:
The key is to recall the standard definitions:

• Isotopes: same number of protons, different numbers of neutrons.
• Isobars: same mass number, different atomic numbers.
• Isotones: same number of neutrons, different atomic numbers.

Isotones therefore share the same neutron count but not the same proton count, which means their atomic numbers are different and they represent different elements. The approach is to identify which option states that nuclides have the same number of neutrons but different atomic numbers, which matches the definition of isotones.

Step-by-Step Solution:
Step 1: Recall that isotones are defined by having equal numbers of neutrons in their nuclei. Step 2: Because the number of protons is not restricted to be the same, the atomic numbers of isotones are different, so they are different elements. Step 3: Examine option A, which states that the number of protons is the same but neutrons differ. That is the definition of isotopes, not isotones. Step 4: Option B claims that both atomic number and mass number are the same, which would mean the nuclides are actually the same, not a special related pair. Step 5: Option C states that the nuclides have the same number of neutrons but different atomic numbers, which matches the definition of isotones. Step 6: Options D and E describe situations that do not match the standard definition of isotones, so they can be rejected.

Verification / Alternative check:
Consider an example. Oxygen 17 has 8 protons and 9 neutrons. Fluorine 18 has 9 protons and 9 neutrons. These two nuclides have the same neutron count, 9, but different atomic numbers, 8 and 9. They are isotones. In contrast, carbon 14 and nitrogen 14 both have mass number 14 but different neutron counts, so they are isobars, not isotones. Chlorine 35 and chlorine 37 have the same atomic number 17 but different neutron numbers, so they are isotopes. This set of examples shows clearly that isotones are defined by sharing neutron count while differing in proton count, as described in option C.

Why Other Options Are Wrong:
Option A describes isotopes, which have the same number of protons and different numbers of neutrons, not isotones. Option B would mean that both atomic number and mass number are the same, which actually describes identical nuclides rather than a special category. Option D again describes identical nuclides, because if both the proton and neutron numbers are the same, the nuclei are not distinct. Option E focuses on electrons in the outermost shell, which is more related to chemical behaviour and valence, not nuclear classification. Only option C correctly states that isotones have the same number of neutrons but different atomic numbers.

Common Pitfalls:
A common difficulty is mixing up the similar sounding terms isotopes, isobars and isotones. Students often remember isotopes but forget isotones, or they confuse isotones with isobars. A helpful memory aid is to focus on the last part of each word: topo suggests place, so isotopes have the same place in the periodic table, meaning same atomic number. Bar refers to weight or mass, so isobars have equal mass number. Tone can be linked to neutrons only, so isotones have equal neutron number. Keeping these associations in mind reduces confusion during nuclear chemistry questions.

Final Answer:
Isotones are nuclides that have Same number of neutrons but different atomic numbers, so they share neutron count but belong to different elements.