For 10 points, name this lightest metalloid, the fifth element.
After one isotope of this element is purified, the waste product is often used as a starting material for aneutronic fusion. It's not gadolinium, but a phenylalanine derivative containing this element is delivered to brain tumors in the proposed mechanism for neutron capture therapy. This element's hydrides adopt closo-, nido-, and arachno- configurations, as predicted by (*) Wade's rules. This element's oxide and silica are found in a ubiquitous glass used in virtually all laboratory glassware. Many compounds involving this element have electron-deficient three-center two-electron bonds because this element, which is present in Pyrex, can only hold on to six valence electrons. For 10 points, name this lightest metalloid, the fifth element.
After one isotope of this element is purified, the waste product is often used as a starting material for aneutronic fusion.
boron [or B]