Sentence:  The <WSD>O</WSD> atoms in each sheet are close packed and each Cr atom is surrounded by a distorted octahedron of O atoms . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust