Concept:Isobars are atoms of different elements that have the same mass number but different atomic numbers.
Explanation:The pair
1840Ar and
2040Ca both have a mass number of
40.
Their atomic numbers are
18 and
20 respectively—these are different.
Since the mass number is the same but atomic numbers differ, these two are isobars.
They are not isotopes because isotopes require the same atomic number.
They are not isotones because the number of neutrons (mass number minus atomic number) is
22 for Ar and
20 for Ca, so neutron counts differ.
They are not isoelectronic because the number of electrons (equal to atomic number for neutral atoms) is also different.
Thus, the correct classification is isobars.
Answer:A. isobars