Partially substituting a compound of silicon and this element yields the refractory material Sialon. This element is at the center of salts used in the first three generations of deep eutectic solvents. This element is the heaviest that can be the X in MXenes (MAX-eens). Traps in Schlenk lines with solutions of this element can be dangerous if they are exposed to oxygen. "Quats" centered around this element are common phase-transfer catalysts. This element is positively charged at the center of the pseudohalogen fulminate. As a prototypical (10[1]-5[1])example, this element lends its name to the fluxional inversion of pyramidal molecules it is in. (10[1])Platinum-rhodium gauze catalyzes the production of this element's oxyacid in the (10[1])Ostwald (10[1])process; that acid is (10[1])the heavier component of (10[1])aqua (-5[1])regia. For 10 points, name this central element of azide and ammonium ions. (10[2])■END■