Chapter 3 introduces four factors that stabilize a conjugate base, sometimes remembered by the mnemonic ARIO — Atom, Resonance, Induction, Orbital (hybridization). Every trend in this appendix is an example of one of these four:

Factor	What It Means	Example From This Appendix
Atom	More electronegative atoms (further right on the periodic table) or larger atoms (further down the periodic table) stabilize negative charge better — electronegativity dominates across a period, size dominates down a group	Underlies why O–H is more acidic than N–H (electronegativity); not heavily featured in this course’s scope otherwise
Resonance	Delocalizing charge across multiple atoms lowers its energy	Carboxylic acids, phenols, and enolates are all far more acidic than their non-delocalized counterparts (alcohols, alkanes)
Induction	Nearby electronegative atoms withdraw electron density through sigma bonds, stabilizing adjacent charge	Trifluoroacetic acid vs. acetic acid; chloroacetic acid vs. acetic acid
Orbital (hybridization)	More s-character holds electrons closer to the nucleus, stabilizing negative charge	Terminal alkyne C–H (sp) vs. alkene C–H (sp²) vs. alkane C–H (sp³)

When comparing two acids, working through these four factors in order — is one atom more electronegative, can the charge resonate, is there an inductive effect nearby, does hybridization differ — will explain nearly every pKa trend in this table without needing to memorize the numbers directly.

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