When was the concept of resonance structures introduced?
The concept of resonance was first introduced in 1899.
in 1899 Johannes Thiele first used his "partial valence hypothesis" to explain the unusual stability of benzene.
In 1926 Werner Heisenberg explained resonance by using quantum mechanics.
Resonance theory wasn’t really accepted by organic chemists until Linus Pauling developed the concept further between 1928 and 1933.