Calcium (atomic number 20) and argon (atomic number 18) have a mass nu

Calcium (atomic number 20) and argon (atomic number 18) have a mass number of 40. What are they known as?

Isotones

Isochores

Isobars

Isotopes

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This question was previously asked in

UPSC CDS-1 – 2017

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The correct answer is C) Isobars.

Atoms of different elements that have the same mass number (sum of protons and neutrons) but different atomic numbers (number of protons) are called isobars. Calcium has atomic number 20 and mass number 40. Argon has atomic number 18 and mass number 40. Since they have different atomic numbers (20 vs 18) but the same mass number (40), they are isobars.

– Isotopes are atoms of the same element (same atomic number) with different mass numbers (different number of neutrons). Example: Carbon-12 and Carbon-14.
– Isotones are atoms of different elements (different atomic numbers) with the same number of neutrons. For Calcium-40, neutrons = 40-20 = 20. For Argon-40, neutrons = 40-18 = 22. They are not isotones.
– Isochores relate to constant volume processes in thermodynamics, not atomic structure.

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