According to the Hume-Rothery rules, solid solutions only form when this quantity differs by less than 15% for solute and solvent. The lattice constant for BCC crystals is four over root three times this quantity. This quantity is smaller for cations than their corresponding neutral atoms. Because f electrons are very poor at shielding, this quantity does not change significantly across the lanthanide series. In general, this quantity decreases from left to right across a period and increases down a group. It equals 0.7 angstroms for carbon, half the length of a carbon-carbon bond. For 10 points, name this quantity measuring the "size" of an 
According to the Hume-Rothery rules, solid solutions only form when this quantity differs by less than 15% for solute and solvent. The lattice constant for BCC crystals is four over root three times this quantity. This quantity is smaller for cations than their corresponding neutral atoms. Because f electrons are very poor at shielding, this quantity does not change significantly across the lanthanide series. In general, this quantity decreases from left to right across a period and increases down a group. It equals 0.7 angstroms for carbon, half the length of a carbon-carbon bond. For 10 points, name this quantity measuring the "size" of an atom.
According to the Hume-Rothery rules, solid solutions only form when this quantity differs by less than 15% for solute and solvent.
atomic radius [or ionic radius; or van der Waals radius; prompt on atomic "volume"; prompt on atomic "size"]