MOLECULESMolecule, smallest unit of a substance that shows all the chemical properties of that substance. A molecule is a group of atoms that are bound tightly together by strong chemical bonds called covalent bonds. Every molecule has a definite size. If a molecule is broken up into its atoms or into smaller groups of atoms by chemical processes, these pieces will not behave like the original molecule. A molecule can contain atoms of the same element or atoms of different elements. A substance made up of molecules that include two or more different chemical elements is called a molecular compound. An example of a molecular compound is water. Water is made of molecules that contain two hydrogen atoms and one oxygen atom.

MOLECULESMany substances on Earth are made of molecules. Millions of molecules join together to make up the cells in humans or in any other plant or animal. The food we eat, the air we breathe, the clothes we wear, and the wood, paint, and carpeting that we use in homes are all made of molecules. Millions of different molecules exist in nature or can be made by chemists. The nature of each molecule depends on the atoms that it contains and how they link to each other. For example, the oxygen that animals require is made of molecules that have two oxygen atoms bound together. If one oxygen atom binds to a carbon atom, the molecule is instead the poisonous gas carbon monoxide.

MOLECULESScientists study molecules and their structures so they can better understand why substances behave the way they do. For example, molecular structure helps explain why water boils at a high temperature. Scientists and manufacturers also use their knowledge of molecules and molecular structures to make substances with desirable properties. Plastics, for instance, are laboratory-made substances that consist of enormous molecules containing thousands of atoms. By manipulating the molecular structure of plastics, chemists have created materials that stretch better, resist fading, or can be used in microwave ovens without melting. Similarly, pharmaceutical chemists use their knowledge of molecular structure to develop new drugs that more effectively ease pain or fight disease. The discovery of the structure of deoxyribonucleic acid (DNA), the molecule that contains the genetic blueprint for living organisms, opened the door to tremendous advances in medicine and industry. Knowledge of the structure of DNA has enabled physicians to understand and treat certain genetic diseases. Moreover, by manipulating DNA structure, scientists have been able to modify—or genetically engineer—organisms, creating, for example, bacteria that produce valuable drugs.

MOLECULES

Although much of our world is composed of molecules, not all substances are molecular. As we will discuss later, metals do not consist of molecules; nor do ionic compounds, which are crystalline substances such as common table salt. The atoms in metals and ionic compounds form different arrangements from those of molecular structures.

MOLECULAR FORMULA

Molecular formulas are a shorthand way of describing molecules and compounds. Chemists use formulas to talk and write about molecules and to indicate how molecules behave in chemical reactions. The molecular formula indicates, in special notation, which elements make up the molecule and how many atoms are needed of each element. Understanding these formulas is the first step toward understanding the language of chemistry.

MOLECULAR FORMULA

Scientists use shorthand symbols for the elements in molecular formulas. These symbols can be found in the periodic table, a chart that arranges the elements according to their chemical properties (see Periodic Law). For example, H stands for hydrogen, C for carbon, and O for oxygen. To indicate a molecule, chemists write the number of atoms of each element in subscript to the right of the symbol. A water molecule, for example, contains two hydrogen atoms and one oxygen atom, and its formula is written as H2O. A molecule of the compound ethane contains two carbon atoms and six hydrogen atoms, giving the molecular formula C2H6. A molecule of butane, C4H10, contains four carbon atoms and ten hydrogen atoms. The molecular formula of a compound is also called its chemical formula. Scientists also use chemical formulas to describe ionic compounds, which contain elements in definite proportions but do not actually contain molecules.

MOLECULAR FORMULA

The empirical formula of a molecule is a simpler formula than the molecular formula. It is useful when scientists know only the ratio of atoms in a compound, for example, after performing a chemical analysis that reveals the weight of each element in the compound. The empirical formula looks similar to the molecular formula, but the subscripts only include information on the ratios of the elements with respect to each other and not on the actual number of atoms. For example, ethane’s molecular formula is C2H6, which shows that the ratio of carbon atoms to hydrogen atoms is 1 to 3, so its empirical formula is CH3. An unknown sample with the empirical formula CH3 may be ethane, but it cannot be butane, which has an empirical formula of C2H5. Water’s molecular formula is the same as its empirical formula, H2O. Molecular formulas always have subscripts that are whole number multiples of the empirical formula of a compound. Chemists also use empirical formulas for ionic compounds.

MOLECULAR FORMULA

The structural formula of a molecule provides even more information than does the molecular formula. It shows which groups of atoms bond to each other in a molecule. Structural formulas help differentiate between isomers, molecules that have the same molecular formula but different structures. For example, C5H12 may represent the substance pentane, with the structural formula CH3-CH2-CH2-CH2-CH3, or it may represent isopentane (also called 2-methyl pentane), with the structural formula CH3-CH2-CHCH3-CH3.

SIZES AND SHAPES

Molecules come in many sizes and shapes. They range in size and complexity from the tiny, diatomic molecules (of which the hydrogen molecule is the smallest) to enormous molecules with thousands and thousands of atoms, such as DNA and plastics molecules. The size and shape of a molecule depends on the number of atoms it contains and how the atoms are arranged. For large molecules, the shape also depends on the flexibility of the molecule. Long chains of atoms can coil up into a variety of shapes.

SIZES AND SHAPES

The size and shape of the molecules in a substance determine many properties of the substance. For example, small molecules tend to separate from each other more easily than larger molecules do, unless other attractive forces are involved. This means that substances made of small molecules usually boil or evaporate into gases at lower temperatures than do substances made of similar, larger molecules. Air is a gas that mainly contains small molecules of nitrogen and oxygen. These molecules boil at extremely low temperatures.

SIZES AND SHAPES

Molecular shape can affect properties such as the elasticity and rigidity of a substance. Shape can also determine how molecules function in living organisms. The shapes of large protein molecules are especially important in animals and plants. Many protein molecules work by fitting together with other molecules, in much the same way that a lock and key fit together. For example, inside your nose are protein molecules shaped to fit with the molecules of particular odors. Certain scent proteins fit with the molecules that give chocolate its odor, while another set of scent proteins fit with the molecules that make bananas smell as they do. Similarly, the protein hemoglobin, which is found in our red blood cells, has a shape that fits exactly with oxygen molecules, enabling the red blood cells to carry oxygen throughout the body. If a protein has the wrong shape, it will not work properly. For example, the disorder sickle-cell anemia results when hemoglobin molecules are deformed and cannot pass through the capillaries readily.

SIZES AND SHAPES

The size and shape of a molecule depend on the type and number of atoms that make up the molecule and how they are arranged. The smallest molecules—such as hydrogen, oxygen, and water molecules—contain only a few atoms. These molecules are smaller than one-millionth of a meter at their widest point. Scientists usually measure them in Angstroms (Å), where one Å is 10-10 (or 1/10,000,000,000) meters. The hydrogen molecule, made of two hydrogen atoms, is about 1.5 Å. The oxygen molecule, made of two oxygen atoms, is slightly larger, since oxygen atoms are slightly larger than hydrogen atoms are.

SIZES AND SHAPES

Many carbon-containing molecules, such as proteins and plastics, are made of long chains of thousands of atoms. Although such molecules are thousands of times longer than the smallest molecules, they are still microscopic in width. Some of the longest natural molecules are the DNA molecules found in the cells of every living organism. The longest human DNA molecule, when fully stretched out, spans about 9 cm (about 4 in). However, DNA molecules twist and curl such that 46 can pack into the microscopic nucleus of a human cell.

DISCOVERY OF MOLECULES

Until the 1800s chemists did not understand the difference between ionic and molecular compounds. They considered anything that contained more than one element to be a compound. Investigators, such as British scientists Michael Faraday and Henry Cavendish, began to differentiate the two when they realized that some compounds, when dissolved in water, made the water conduct electricity more easily, while others did not. Cavendish gave himself electric shocks to measure the conductivity of these water solutions. His results were surprisingly accurate.

DISCOVERY OF MOLECULES

Dutch chemist Jacobus Hendricus Van’t Hoff (who received the first Nobel Prize in chemistry in 1901) and Swedish chemist Svante August Arrhenius explained why different water solutions conduct electricity differently. Van’t Hoff determined that salts—such as sodium chloride (NaCl), or table salt, and potassium chloride (KCl)—split into two particles when they dissolve in water, while substances such as glucose do not split apart when they dissolve. Arrhenius realized that the dissolved salts not only split, but they split into two electrically charged particles, or ions. The ions move through the water to conduct electricity. Substances such as glucose do not split and thus dissolve into uncharged compound particles that do not conduct electricity, that is, into molecules.