How Do Atoms Combine with Each Other

A couple of millennia ago, the Greeks believed that water was one of the four elements of nature, along with fire, air, and earth, and that all things were made from combinations of these elements. Today, we of course know that there are more than a hundred elements. And, in fact, water is not a single element but a combination of atoms of two elements, namely hydrogen (H) and oxygen (O). When two or more atoms of the same or different elements combine together, molecules are formed. Therefore, water is a molecule. The chemical formula for a water molecule (H2O) is probably the most widely quoted of all chemical formulas. A chemical formula is merely a molecule's atomic recipe. Thus, for each molecule of water, two hydrogen atoms (subscript 2 behind H) are bound to one oxygen atom (no subscript, so 1 is implied).

From our previous description of the size of atoms you can imagine then that an ordinary glass of water must contain millions of water molecules. In fact, we can use water to tidy up our understanding of elements, atoms, and molecules. If we have an 8 ounce (oz) glass of pure water, we can say that the container is accommodating millions of molecules of water, and thus millions of atoms; however, only two elements are present, oxygen and hydrogen.

Atoms can link together or bond by two means. First, charged atoms can interact with oppositely charged atoms. Remember, as in so many aspects of life, opposites attract. Perhaps the best example of this kind of bonding is sodium chloride (NaCl) or common table salt. Here, the negatively charged chloride ions (Cl-) are attracted and electrically stick to positively charged sodium ions (Na+). You can also check your toothpaste for sodium fluoride (NaF) or toothpaste salt. By the way, the term salt is a general term that describes these types of electrical interactions.

Na+y Cl- sodium chloride (table salt) Na+y F- sodium fluoride (toothpaste salt)

Another way that atoms can bond with each other is by sharing electrons. This is a fascinating event whereby atoms share electrons between them to form a stable union. In Figure 1.3 and throughout this book you will see a straight line connecting atoms that are bonded in this manner. Probably the best examples of this type of bonding are the so-called organic molecules, which refers to those molecules that contain carbon atoms. Organic also refers to that which is living. Therefore, the most important molecules of life must be carbon based. In fact, a large portion of this book discusses organic molecules, such as proteins, carbohydrates, fats, cholesterol, nucleic acids, and vitamins.

What Is the Design of Molecules?

One limitation of an ink-and-paper representation of molecules is that it often fails to truly capture the three-dimensional beauty of molecules. For example, DNA molecules exist in a spiral staircase design, while many protein molecules appear to be all bunched (or "globbed") up. The three-dimensional design of a molecule helps determine what that molecule can do (its properties). Furthermore, we will see that many of the important molecules in our body are actually combinations of smaller molecules. For instance, proteins are made from amino acids, and fat molecules are made from fatty acids and glycerol.

Methane (CH4) Water (HzO) Carbon dioxide (C02)

Figure 1.3 Methane (CH4) and carbon dioxide (CO2) are organic molecules while water (H2O) is not.

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