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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.
Nuclear chemistry requires an understanding of the atomic nucleus, which we've already explored a bit in this guide. Nuclear chemistry also involves explosions, although nuclear explosions are of much larger proportions that anything we see on YouTube. Yikes.
Interestingly, nuclear reactions are not the result of changes in chemical bonding. Instead, nuclear chemistry is rooted in changes in individual atoms—the nucleus of the atom to be exact. Seeing where nuclear chemistry gets its name? Here are a few different types of nuclear reactions that we will explore later in greater detail.
Nuclear fusion refers to when subatomic particles come together, or fuse. The result is a ton of energy, and we mean a ton. Want an example? Take a look at the sun. Well, actually just think about the sun—we want to protect our eyeballs, especially after we go through all the trouble of wearing annoying safety goggles in lab. The energy generated from the sun is an example of nuclear fusion. On the sun, four protons fuse together to make helium atoms and in the process, energy is liberated. Who ever said there isn't such a thing as free energy?
Nuclear fission occurs when atoms with large atomic masses split apart. We are talking about mass numbers of 230 or more; these are the big guys of the periodic table3. However, these reactions are slow, even by chemistry standards. If you had a sample of uranium, it would take 10,000,000,000,000,000 years for half of the sample to undergo nuclear fission3. Chemists use nuclear reactors to induce nuclear fission reactions so that they don't have to wait around so long. Seriously, some science experiments take forever.
The process of unstable atoms breaking down and releasing energy is called radioactive decay. There are a few different types of decay: alpha decay, beta decay, gamma decay, and tooth decay. (One of those isn't quite right.) The other types of decay aren't too good for you, either.
Your skin easily blocks alpha particles, released by alpha decay. However, if you should ingest alpha particles, that's a real problem. Read more about the fatal poisoning of a spy by polonium. A sheet of aluminum can stop beta particles, but they are also potential hazards as beta particles can mutate DNA. Mutated DNA leads to things like Teenage Mutant Ninja Turtles walking around the streets...scary. Gamma rays are the most harmful as they are very high energy, and it takes a block of lead to stop them from penetrating the body. Ever wonder why the dentist puts a lead apron on your body right before taking X-rays of your teeth? (We knew this would come full circle.) The answer is that the apron is there to protect your body from the X-rays, which would otherwise needlessly expose your body to extra gamma radiation.
We promise we will return to the land of nuclear reactions in a later section. We have just scratched the surface. There's still the topic of radioactive dating to explore. No, we don't mean dinner and a movie.