boron (B) /bôr″on/ , a nonmetallic element, whose atomic number is 5; its atomic mass is 10.81. Elemental boron occurs in the form of dark crystals and as a greenish yellow amorphous mass. Certain concentrations of this element are toxic to plant and animal life, but plants need traces of boron for normal growth. It is the characteristic element of boric acid, which is used chiefly as a dusting powder and ointment for minor skin disorders. Boric acid in solution was formerly extensively used as an antiinfective and eyewash, but the high incidence of toxic reactions and fatalities associated with these preparations has greatly reduced their use.