In the study of Atomic Structure, scientists realized that not all atoms follow the normal rules of the periodic table. Sometimes, two completely different elements can surprisingly weigh exactly the same amount.
These strange twin atoms are called Isobars.
Definition: Different elements, different atomic number, but the exact SAME Mass Number.
Greek Origin: The word comes from 'Iso' (Equal) and 'Baros' (Weight).
Chemical Properties: Because they are different elements with different electrons, their chemical reactions are completely different.
Classic Exam Example: Argon-40 (₁₈Ar⁴⁰) and Calcium-40 (₂₀Ca⁴⁰).
Isobars are defined as atoms of completely different chemical elements that have a different Atomic Number (different number of protons), but they share the exact same Mass Number (same total weight of protons + neutrons in the nucleus). Because they are completely different elements, their chemical properties and reactions are 100% different. The only thing they share in common is their physical weight.
The ultimate classic example asked in every chemistry exam is the pair of Argon (Ar) and Calcium (Ca):
Students constantly confuse these two terms. Memorize this strict difference:
Isobars are atoms of completely different chemical elements that have different atomic numbers but share the exact same mass number.
The most famous example is Argon (atomic number 18) and Calcium (atomic number 20). Despite being completely different elements, they both have an exact mass number of 40.
Isotopes are atoms of the SAME element that have different weights. Isobars are atoms of DIFFERENT elements that have the exact same weight.
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