Physics Nuclear Physics MDCAT 2013
PMDC Verified Question 78 of 98
Isotopes are those nuclei of an element that have:
A
Same mass number but different atomic number
B
Different mass number as well as atomic number
C
Same mass number as well as atomic number
D
Same atomic number but different mass number
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Same atomic number but different mass number


Concept:

Elements are defined by their atomic number (number of protons). Isotopes represent different physical variants of the same chemical element.

Solution:

  • Because they are the same element, they must have exactly the same atomic number (\(Z\)).


  • However, they contain different numbers of neutrons in their nuclei.


  • Therefore, their total nucleon count differs, resulting in a different mass number (\(A\)).


Why other options are incorrect:

Option A defines isobars. Option B defines entirely different elements. Option C defines identical nuclei.

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