It is important to note that A-values do not predict the physical size of a molecule, only the steric effect.  For example, the tert-butyl group (A-value=4.9) has a larger A-value than the trimethylsilyl group (A-value=2.5), yet the tert-butyl group actually occupies less space. This difference can be attributed to the longer length of the carbon–silicon bond as compared to the carbon–carbon bond of the tert-butyl group. The longer bond allows for less interactions with neighboring substituents, which effectively makes the trimethylsilyl group less sterically hindering, thus, lowering its A-value. This can also be seen when comparing the halogens. Bromine, iodine, and chlorine all have similar A-values even though their atomic radii differ. A-values then, predict the apparent size of a substituent, and the relative apparent sizes determine the differences in steric effects between compounds. Thus, A-values are useful tools in determining compound reactivity in chemical reactions.