Molecules form when individual atoms create bonds by sharing electrons. Understanding how atoms combine to make molecules allows scientists to predict many of the physical and chemical properties of substances. Since the outermost eight electrons are key to forming compounds, this unit shows how the Octet Rule provides a basis for predicting how atoms may gain, lose, or share electrons to fill the slots in their outer shells. A fundamental understanding of how electrons form bonds leads to the three-dimensional shapes of molecules and has implications in all aspects of chemistry. Part of the series Chemistry: Challenges And Solutions.