To make sense of the σ values, we need to look in more detail at how substituents can stabilize a developing positive or negative charge. Two factors are involved. The more obvious is the effect of the electronegativity of the substituent. Carbon and hydrogen have similar electronegativities, so C-H and C-C bonds in neutral saturated organic molecules are non-polar. Carbon becomes more electronegative in CH3 as it changes its hybridization and acquires a positive charge. If one hydrogen is replaced by a fluorine atom, the electronegative fluorine atom will polarize the sigma bond so that, relative to the C-H bond, the electron density will be centered closer to the fluorine, and away from the C