What defines two elements as isobars?
Same mass number but different atomic numbers
Isobars are atoms of different elements that have the same mass number (total protons + neutrons) but different atomic numbers (different number of protons). For example, argon-40 (Z = 18) and calcium-40 (Z = 20) both have mass number 40, yet they are distinct elements.
"Same number of electrons" describes a neutral atom only in terms of its protons; equal electron count would mean the same atomic number, which would make them the same element, not isobars.
"Same number of neutrons" defines isotones, not isobars.
"Same atomic number" defines isotopes (same element, different mass numbers), which is the opposite condition.
Hence, two elements are isobars when they have the same mass number but different atomic numbers.
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