Sentence:  Hence it is difficult to conceive of a packing of the atoms in this material in which the <WSD>oxygen</WSD> atoms are far from geometrical equivalence . 
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