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|Chapter Two: The Chemistry of Biology|
Everything, whether it be a giraffe, a plastic cup, your computer, or the air, is made of what is called matter. Matter is just the "stuff" that everything, absolutely everything, is made of. Certain types of matter can be broken down into simpler pieces of matter, some of which can be broken down again and again. The process of breaking down (or building up) matter is performed by a chemical reaction; that is, when two substances (chemicals) come together and react with one another. Why this occurs will become clear to you later.
When matter is broken down into simpler and simpler parts, there comes a point when chemical reactions can no longer split the matter. This special type of matter that cannot be further broken is called an element. For example, oxygen, hydrogen, carbon, and sodium are the names of some commonly known elements. Each element has a one or two letter symbol which is used by scientists to identify it. Using the previous examples, oxygen is O, hydrogen is H, carbon is C, and sodium is Na.
Currently, over one hundred elements have been discovered, and each has its own distinct properties. Any amount of a particular element, whether it be the size of a car or the size of a pea, will always exhibit those same properties. The smallest amount of an element that you can have that still exhibits these properties is called an atom, the unit of structure of matter.
Atoms are very, very small. Even the most powerful microscopes can only picture atoms as tiny white dots! Since observing atoms directly is nearly impossible, scientists have, over the years, determined the basic structure of an atom through experimentation. All atoms share a similar structure. In the center is a nucleus, a ball of two types of particles called protons and neutrons. Both of these particles have the same mass (one atomic mass unit), but the proton has a positive charge while the neutron has a neutral charge. Similar to how the planets orbit the Sun, tiny particles called electrons orbit the nucleus. Electrons are much smaller than protons, but they happen to have a negative charge exactly equal to the positive charge of a proton. Normal atoms contain the same number of protons and electrons, so the positive and negative charges cancel, leaving the entire atom with neutral charge.
What makes atoms different from one another is the number of protons, neutrons, and electrons that they contain. All atoms of a particular element have the same number of each of these particles. Scientists have therefore given each element an atomic number, which is simply equal to the number of protons in an atom of that element. For example, all atoms of the simplest element, hydrogen, have one proton and one electron. The atomic number of hydrogen is 1. A sodium atom has eleven protons and twelve neutrons in the nucleus, which is orbited by eleven electrons. The atomic number of sodium is 11. The atomic mass of an element is just the number of protons plus the number of neutrons it has. So, the atomic mass of hydrogen is 1 (1 proton + 0 neutrons = 1), and the atomic mass of sodium is 23 (11 protons + 12 neutrons = 23).
An important point to remember is that positive and negative charges attract to one another, while two negative charges or two positive charges repel one another. Have you wondered why there are only about a hundred elements? Well, protons in the nucleus of an atom tend to repel one another (because they all have a positive charge), so the nucleus is slightly unstable. In elements with a great many protons, the force of repulsion is so great that the nucleus breaks apart. Although over a hundred elements have been discovered, many of them must be artificially created in a laboratory, and they quickly break apart once they are formed.
Certain elements come in several forms, but each of these forms differs from the first only in the number of neutrons in the nucleus. These separate forms are called isotopes of that element. For instance, hydrogen has two isotopes, deuterium and tritium. Normally, hydrogen has no neutrons, but deuterium has a single neutron in the nucleus, and tritium has two neutrons.