Outer Shell

• The outer shell of electrons is the part of an atom that is engaged in chemical reactions. Molecules are driven to react in such a way as to complete as many atoms' outer shells as possible, because it maximizes stability.
  
  

Periodic Table

• Noble gases in orange; yellow and gold combine easily
  
  The pattern of completing outer shells is seen in the structure of the periodic table. For example, the noble gases in the far-right column are chemically inert because their outer shells are full. Elements just above and just below a noble gas's sequence position are highly reactive with each other.
  
  

Energy and Entropy

• Most spontaneous reactions are exothermic, i.e., they give off more heat than they absorb. It is similar to a skier going down a hill, where the gravitational potential energy is lower. The drive to achieve minimum energy and maximum entropy is what motivates chemical reactions.
  
  

Free Energy

• The interaction of these two drives is the motivation for Willard Gibbs' invention of "free energy," a mathematical construct incorporating both entropy and heat to more completely analyze reaction spontaneity.
  
  

Acids and Bases

• One important kind of chemical reaction is neutralization. This is when an acid and base combine to neutralize each other. Neutralization reactions generally produce a salt (molecule with an ionic bond) and water, an example being HCl(aq)+NaOH(aq) ' NaCl(aq)+H2O.