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Calcium-40 (Z=20, A=40) and Argon-40 (Z=18, A=40) are best described as:
AIsobars, with the same mass number but different element
BIsotopes, both being heavy forms of the same element
CAllotropes, two crystal forms of the same chemical
DIons, both carrying the same net electric charge
Answer & Solution
Correct answer: A. Isobars, with the same mass number but different element
Isobars have the same mass number A but different atomic numbers Z (and so are different elements). Calcium-40 and Argon-40 are classic isobars: same A=40, different Z (20 vs 18).
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