In the Goldschmidt correction, relative factors used to adjust this quantity are based on 12-fold coordinated close-packed structures. A different quantity named for Allred and Rochow equals 3590 times Z-effective over this quantity squared. The (-5[1])Hume-Rothery rules state a substitutional alloy will only form if this quantity for two elements is within 15% of each (15[2])other. A “hard sphere” model can be used to calculate the (*) van der Waals form of this (10[1])quantity. Poor (10[1])shielding against nuclear charge by 4f electrons explains why this quantity “contracts” for (10[1])lanthanides. Like metallic (10[1])character, this quantity decreases from left to right across a period and increases down a group. For 10 points, an atom’s size is measured by what distance between an atom’s nucleus and outermost (10[1])electron? ■END■