Organic Chemistry MCAT Practice Exam 2025 – The All-in-One Guide to Master Your Test Preparation!

A major resonance contributor in a molecule is primarily defined by the fulfillment of the octet rule. In resonance structures, these contributors demonstrate the most stable arrangement of electrons, typically where atoms, particularly second-period elements like carbon, nitrogen, and oxygen, achieve full valency by having eight electrons in their valence shell.

When assessing resonance structures, those that adhere to the octet rule are often more stable and contribute more significantly to the resonance hybrid, as they minimize formal charges and allow for more effective overlapping of atomic orbitals. This stability is critical, as resonance is a way to depict the delocalization of electrons within a molecule and reflects how the molecule behaves in reality.

The presence of a strong base relates to the reactivity conditions, while stability without resonance can indicate that the molecule itself is not participating in resonance, making it irrelevant in determining the significance of a resonance contributor. High electronegativity of atoms affects the distribution of electron density but does not directly define contribution in resonance structures. Thus, the presence of stable resonance structures that adhere to the octet rule plays the most crucial role in defining major contributors.