S: (n) couple ((physics) something joined by two equal and opposite forces that act along parallel lines)
S: (n) unit cell (the smallest group of atoms or molecules whose repetition at regular intervals in three dimensions produces the lattices of a crystal)
S: (n) group, radical, chemical group ((chemistry) two or more atoms bound together as a single unit and forming part of a molecule)
S: (n) molecule ((physics and chemistry) the simplest structural unit of an element or compound)