Concept:Isotopes are variants of a particular chemical element which share the exact same atomic number (number of protons) but differ in nucleon number (mass number, meaning a different number of neutrons).
Formula:$$ \text{Isotopes MUST have: Same } Z \text{ (subscript), Different } A \text{ (superscript)} $$
Solution:- Let's evaluate the standard notation \( ^{A}_{Z}E \), where Z is the bottom number (protons) and A is the top number (mass).
- Option D: Shows \( ^{14}_{6}X \) and \( ^{15}_{6}Y \). Both share the same atomic number \( Z=6 \), but have different mass numbers (14 and 15). These are two distinct isotopes of Carbon.
Why other options are incorrect:- Option A (\( ^{12}_{6}X \) and \( ^{12}_{7}Y \)): Different atomic numbers (6 and 7). These are Isobars, not isotopes.
- Option B (\( ^{16}_{8}X \) and \( ^{16}_{8}Y \)): Same atomic number AND same mass number. These are completely identical atoms, not two different isotopes.
- Option C (\( ^{18}_{9}X \) and \( ^{20}_{10}Y \)): Different atomic numbers and different mass numbers. They are unrelated elements.
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