For every element, there is an internationally agreed symbol that can be substituted for its name. The simplest symbols are the first letter of the name. Thus, the elements hydrogen, carbon, and oxygen are abbreviated H, C, and O.
There are many more elements than letters of the alphabet. Thus, most elements have to have two-letter symbols. Chlorine is the next heaviest element after carbon that begins with the letter C. It is designated CI. Calcium is Ca. Cadmium, with the first two letters the same as in calcium, has the symbol Cd. (See the table on page 15 for a list of all the elements with their symbols.)
Most of the symbols for the elements are taken from their English names. However, in some cases, the symbols come from the Latin. Thus, sodium has the symbol Na (natrium).
The composition of simple molecules can be shown by combining the symbols for the elements that the molecules contain. Hydrogen fluoride is a molecule that contains one atom of hydrogen and one atom of fluorine. It can be written as HF. In many molecules, more than one atom of the same element is found. The number of atoms of any one element in a molecule is shown by a subscript number. Such a number is written after the symbol and slightly below the line. An example is the hydrogen molecule, which is written H2. A water molecule contains two hydrogen atoms and one oxygen atom. It is represented as H20.
The hydrogen molecule is called homoa-tomic (homogeneous). It is made from atoms of the same element. If a hydrogen atom joins with a fluorine atom, a molecule of hydrogen fluoride results. Such a molecule is called heteroatomic (heterogeneous). It is composed of atoms from two (or more) different elements.
Atoms unite together to form molecules and compounds by means of chemical bonds. How the atoms bond together depends on how the electrons interreact. Electrons, orbitals, and the roles they play in chemical reactions are discussed in the preceding article. There are three types of chemical bonds that combine atoms into molecules and compounds. These bonds are covalent, ionic, and coordinate. In a covalent bond between two atoms, electrons from both atoms are shared by both nuclei. The electrons can be shared equally. An example is the hydrogen molecule, which consists of two hydrogen atoms. The two electrons (one from each atom) revolve around both nuclei equally. The same amount of time is spent around each nucleus. Such a molecular orbital has a balanced shape. The electrons in a covalent bond can also be shared unequally. For example, a hydrogen fluoride molecule consists of one hydrogen atom (with one electron) and one fluorine atom (which has nine electrons). Two electrons (one from each atom) form the covalent bond. But these electrons spend more time closer to the fluorine nucleus than to the hydrogen nucleus. As a result, the path formed by the molecular orbital is pear-shaped. The larger end is around the
Atoms, elements, and molecules: Chemical symbols and chemical bonding 15
To a diner, the salt(far left) is a condiment to put on food. To the road-builders (nearleftl, sand has to be excavated to make a roadbed. Salt and sand are chemical compounds that a chemist calls sodium chloride and silica. Each molecule of salt contains a sodium atom combined with a chlorine atom. Each silica molecule is made up of a silicon atom in chemical combination with two oxygen atoms.
fluorine nucleus. Page 13 has a diagram illustrating these two kinds of covalent bonds.
In an ionic bond, one atom loses one or more electrons to another. Both atoms thus become electrically charged ions. For example, sodium is electropositive. This means that it tends to lose an electron. Fluorine is electronegative—it tends to gain electrons. When the sodium loses its electron, it becomes a positively charged atom, called a positive ion. When fluorine gains an electron, it becomes a negatively charged atom, called a negative ion. The two atoms thus unite, the sodium atom losing one electron to the fluorine atom. The compound sodium fluoride is then formed.
A coordinate bond is the same as a covalent bond, except that both electrons come from one of the atoms, rather than one electron from each atom. This type of bonding is very important among certain types of metals known as transition elements.
Isomerism is a phenomenon in which the same number and types of atoms may join together in different molecular arrangements. In other words, two compounds may have the
exact same parts. Yet, because their structural arrangements are different, the two compounds do not resemble each other. Such compounds (or molecules) have the same chemical composition but occur in distinct forms. These forms are known as isomers.
Besides being bonded in different ways, some atoms can also be joined by more than a single bond. Such multiple bonds happen most frequently with carbon atoms. Isomers also result from the fact that compounds and molecules exist in three dimensions, not in two. This enables the same atoms and molecules to have different spatial arrangements. Optical isomerism is one special type of isomerism. Two molecules of a substance are mirror images of each other. They occur, again most frequently, in carbon chemistry. Isomers are discussed in more detail in the Organic chemistry section.
Examples of isomers are normal butane and isobutane. They are both types of the gas butane. Each isomer has the same number and types of atoms: 4 atoms of carbon and 10 atoms of hydrogen. However, these same atoms are arranged differently in each isomer. As a result, one chemical difference is in their boiling points. Normal butane boils at —0.5° C. Isobutane boils at —12° C.
The tablebelow lists alphabetically the chemical elements and gives their symbols. These symbols are used in chemical formulas and in the periodic tables in the next section of this book.