Lewis acidity is customarily gauged by comparing the relative magnitude of coordinate covalent bonding energies, where the Lewis acid moiety is varied and the Lewis base is kept constant. However, the prediction of Lewis acidity from first principles is sometimes contrary to that suggested by experimental bond energies. Specifically, the order of boron trihalide Lewis acidities predicted from substituent electronegativity arguments is opposite to that inferred by experiment. Contemporary explanations for the divergence between theory, computation, and experiment have led to further consternation. Due to the fundamental importance of understanding the origin of Lewis acidity, we report periodic trends for 21 boron Lewis acids, as well as their coordinate covalent bond strengths with NH3, Utilizing ab initio, density functional theory, and natural bond orbital analysis. Coordinate covalent bond dissociation energy has been determined to be an inadequate index of Lewis acid strength. Instead, acidity is measured in the manner originally intended by Lewis, which is defined by the valence of the acid of interest. Boron Lewis acidity is found to depend upon substituent electroneogativity and atomic size, differently than for known Bronsted-Lowry periodic trends. Across the second period, stronger substituent electronegativity correlates (R-2 = 0.94) with increased Lewis acidity. However, across the third period, an equal contribution from substituent electronegativity and atomic radii is correlated (R-2 = 0.98) with Lewis acidity. The data suggest that Lewis acidity depends upon electronegativity solely down group 14, while equal contribution from both substituent electronegativity and atomic size are significant down groups 16 and 17. Originally deduced from Pauling's electronegativities, boron's substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater a bond (not pi bond) orbital overlap between boron and larger substituents increase the electron density available to boron's valence, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel explanation of boron Lewis acidity, consistent with first principle predictions. The findings resolve ambiguities between theory, computation, and experiment and provide a clearer understanding of Lewis acidity.