Quick Quiz: Nouns Score: Change category, Flip order Reset Correct! Noun: atomic number 15 Your choice? a multivalent nonmetallic element of the nitrogen family that occurs commonly in inorganic phosphate rocks and as organic phosphates in all living cells; is highly reactive and occurs in several allotropic forms a soft silver-white or yellowish metallic element of the alkali metal group; turns yellow in air; occurs in celestite and strontianite a lustrous gray strong metallic element resembling titanium; it is used in nuclear reactors as a neutron absorber; it occurs in baddeleyite but is obtained chiefly from zircon a hard gray lustrous metallic element that is highly corrosion-resistant; occurs in niobite and fergusonite and tantalite