Chemistry is all about chemical reactions. Our understanding of atomic composition and bonding will allow us to later predict how elements will "act" towards one another. The behavior of atoms of any given element is the basis for chemical reactivity. We will later be looking at these reactions from an energy perspective in our thermochemistry section.
Thermochemistry is the branch of chemistry that takes into account the "cost" of atoms coming together or breaking apart. Energy will play a role in our calculus of chemical reactions. Oops, did we say calculus? Fret not. There is no calculus involved in our chemistry. That is just a scary saying.
There are a few things that have to happen in order for a chemical reaction to occur. For starters, the atoms have to get close enough together before thinking about forming a bond. Think of it this way—there is an "energy cost" associated with getting close to another student in one of your classes... that smelly guy who just came back from gym has a high energy cost associated with him. On the other hand, that popular guy or gal who smells quite nice has a low energy cost associated with him/her.
There is also energy that is stored in chemical bonds. This energy will be important for our calculus, err, calculations, as well. Some bonds store tons of energy while others store very little. A useful analogy for bonds is relationships. Think of that couple that is always fighting. Their bond, or relationship, contains lots of energy. When they break up, it will no doubt be an explosive event with lots of drama, tears, and perhaps even a shouting match in the school hallway. Some chemical reactions are like this...minus the tears. Time for a quick break: check out this exothermic reaction.
In an exothermic reaction, energy is released into the environment as a result of the reaction. While some energy is required to break apart the bonds, much more is released in the process. This is the energy that is said to be "stored" in the bonds. Think: explosion.
Now, back to our relationship analogy. Other relationships just seem to fizzle away—no tears, no drama, and certainly no shouting. Not much energy appeared to be "stored" in the couple's bond.
In chemistry, the opposite of an exothermic reaction is an endothermic reaction. In an endothermic reaction, more energy is actually absorbed in the process of breaking a bond than is released. As a result, we don't see explosions with endothermic reactions. Here is a cool endothermic reaction though. Instead of generating heat, this reaction sucks in heat from the surroundings.
Once we get more into thermochemistry, we will take a closer look at exothermic vs. endothermic in terms of thermal transfers. This refers to how energy is transferred during a particular physical change or chemical reactions. Too curious to wait? Check out thermal transfer, the transfer of energy, between objects in this video where a banana is used as a hammer.