A molecule is defined as the smallest particle of an element or a compound that possesses the properties of the original substance. Some materials, such as helium, neon, and argon, tend to form monoatomic molecules. Most molecules, however, are made of two or more atoms. Many gases, such as oxygen, nitrogen, chlorine, and carbon monoxide, are diatomic (meaning they are composed of only two atoms). Triatomic molecules (which are composed of three atoms) include ozone (O3) and carbon dioxide (CO 2). In complex molecules, such as proteins, the number of atoms can range up to hundreds or thousands. Molecules vary in size from less than 1 millimicron to more than 500 millimicrons and in weight from 4 units (for helium) to 40 million units (for the tobacco mosaic virus).
Molecules differ from chemical compounds because compounds consist of different elements. Combinations of the same atoms form molecules, not compounds. For example, when two atoms of oxygen combine they form a molecule of oxygen, not a compound. Therefore, while every compound is a molecule, not every molecule is a compound. Each molecule of a given substance contains the same number and kinds of atoms which are held together by chemical bonds. The two basic types of chemical bonds are covalent and ionic. A covalent bond holds atoms together by sharing electrons; an ionic bond holds them together through electron transference. The nature of these bonds determine the physical form of the molecule. Gases have weak attractive forces between molecules, whereas liquids and solids have stronger forces.
These bonds may be broken or rearranged in a number of different chemical reactions which fall into four basic categories. These are known as combination, decomposition, single replacement, and double replacement reactions. Specific types of reactions in each of these categories include oxidation, reduction, ionization, combustion, polymerization, hydrolysis, condensation, and rearrangement reactions.
In certain cases, atoms within a molecule may be arranged in different configurations to form molecules which contain the same number and same type of atoms, but which have different properties because of their spacial arrangement. Such molecules are known as isomers. Examples of isomers include ethyl alcohol and methyl ether which both contain one oxygen atom, two carbon atoms, and six hydrogen atoms. Other types of isomers include stereoisomers, geometric isomers, and optical isomers.
One important characterization of a molecule is its molecular weight. Molecular weight is the sum of the atomic weights of the atoms in a molecule. For convenience, the chemical formula is used to add up these values. For gasses, a more accurate molecular weight can be determined by measuring the volume of a given weight and then calculating the weight of 22.4 L at 0C and 760 mm of mercury.
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