Molecule

All molecules have a definite mass and size that are dependent on the atoms from which the molecule is made. The mass is equal to the sum of the masses of all the individual atoms in the molecular structure. The size is not only dependent on the atomic components of the molecule, but also on the arrangement of the atoms within the molecule and how tightly they are joined together.

When atoms join other atoms to form molecules, the chemical and physical properties of the compounds are different from those of the elements from which they were formed. These include such things as color, hardness, conductivity, state (solid, liquid, gas), etc. When letters are used to form new words, such as dog, God and good from the letters g, o and d, the meanings of the new words cannot be discovered by observing or studying either the letters g, o and d, or the other words. The new words formed have new and different meanings. This is also true when new molecules form. The properties of the new substances cannot be found by studying the properties of the elements from which they formed or the properties of other similar molecules.

The molecular formula for sugar is C₆H₁₂O₆, which indicates that a sugar molecule is made up of six atoms of carbon, 12 atoms of hydrogen, and six atoms of oxygen. Carbon is an element that is black and often is found in powder form. It is well known as the major component of coal or the black that appears on burnt Figure 3. Structural formulas help differentiate between substances that share identical molecular formulas, such as ethyl alcohol and methyl ether. Illustration by Hans & Cassidy. Courtesy of Gale Group. toast. Pure hydrogen is a the lightest gas known and pure oxygen is a gas present in the air and needed for living things to breathe and for fires to burn. When these three substances chemically combine in the ratio of C₆H₁₂O₆ to form sugar, which is a white, crystalline solid that has a sweet taste and is soluble in water, the properties of the sugar are unlike those of the pure elements from which it was formed. However, if sugar is reacted under extreme conditions (a chemical change), the original substances, carbon, hydrogen, and oxygen could be recovered.

Hydrogen and oxygen atoms can join to form water molecules. Once again, the properties of water (often used to extinguish fires) are completely different from the properties of oxygen gas (needed to support burning). These same two elements, hydrogen and oxygen, also form another common substance, hydrogen peroxide, with a molecular formula of H₂O₂. Hydrogen peroxide, in its undiluted form, can cause serious burns. When diluted, it is often used for bleaching and as an antiseptic in cleansing wounds. These properties are completely different from the properties of the elements from which it is made, hydrogen and oxygen, as well as from the similar molecule, water. You could not boil potatoes in H₂O₂ instead of H₂O without deadly effects. The properties of hydrogen peroxide differ greatly from those of hydrogen, oxygen, and water because each of the substances has its own specific number, kind, and arrangement of atoms.

While the molecular formula gives the basic information about what atoms are joined together and how many of each kind of atom, this formula does not give the whole story. The arrangement of the atoms within the molecule must also be considered since different arrangements of the same atoms within a molecule produce different substances. The molecular formulas for ethyl alcohol (formed from the fermentation of grains and fruits and present in all wines and liquors) and for methyl ether (sometimes used in refrigeration but not the same ether used as an anesthetic) are identical, Figure 4. The electron sea model, (a) represents an alkali metal with one valence electron and (b) represents an alkaline earth metal with two valence electrons. Illustration by Hans & Cassidy. Courtesy of Gale Group. C₂H₆O. However, the chemical properties are very different. This is because the atoms are arranged differently within the molecule. Molecular formulas cannot convey this information. Different kinds of formulas, called structural formulas, are needed to show molecular arrangements. In the case of ethyl alcohol, the oxygen atom is joined to a carbon atom and to a hydrogen atom. But in the case of methyl ether, the oxygen atom is joined to two carbon atoms. This different arrangement of atoms within the molecule is responsible for imparting different chemical properties to the compound. Thus, the new chemical properties are not only dependent on how many of each kind of atom have joined together, but on how these atoms are arranged within the molecule.

Much of the research in the field of chemistry today involves the formation of new substances by trying to change atoms within a molecule or the arrangement of the same atoms. This latter is a particularly important area when the arrangement of the atoms is changed from what is called a right-handed molecule to a left-handed molecule or vice-versa. Molecules such as these behave as they do because of their shape, much like gloves fit either the right or the left hand because of their shape. Some of the current research on new fat-free commercial products, such as ice-cream or cooking oil, involves right- or left-handed versions of the original fat molecules. These mirror-images of the original molecules cannot be absorbed or used by the body because of the different shape of the molecule. Yet because they are often so similar, enough of the original properties of the fat remain intact to make it useful as a substitute. So far, the taste and their inability to withstand high temperatures in cooking are problematic.

Similarly, many powerful drugs in use today have both right- and left-handed versions. Most drugs contain both forms of the molecules because this mixture is cheaper and easier to produce. However, often only one version gives the desired effect while frequently the other produces unwanted side effects. Although it is chemically much more difficult and more expensive to produce drugs of only one "handedness," patients needing these drugs are finding the purer, single-handed version much easier to tolerate. Drug companies are beginning to pay attention and research is being done to produce drugs of only one "handedness" at a reasonable cost.