Formula, Chemical
Each chemical element has a chemical symbol and a combination of chemical symbols gives a chemical formula.
The chemical formula shows what elements are present in a substance. If a pure element is present the chemical formula is merely the chemical symbol for the element. This situation is encountered most often with the noble gases. In some cases, representing an element by its chemical symbol is technically inaccurate but the symbol is used as a shorthand convention. For example, sodium is normally represented by the symbol Na, although in natural situations sodium is encountered as a giant metallic lattice with many sodium molecules bonded together. Each sodium molecule is held in place adjacent to the next molecule by weak intermolecular forces. This produces a large sheet of identical repeating units of sodium. Many layers are joined in a similar manner. Some chemicals are generally encountered as a diatomic bonded pair. For example, oxygen when encountered as a gas has the chemical formula of O2. The halogens are also generally encountered as diatomic molecules.
The chemical formula of an ionic compound is simply the whole number ratio of the positive to negative ions encountered in the compound. Consequently table salt, sodium chloride, has the chemical formula NaCl. This is because for every positively charged sodium ion there is one negatively charged chlorine ion. The overall electrical charge on the compound must be neutral and the positive and negative charges must balance each other. An ionic compound such as sodium chloride exists in a similar manner to the sodium previously described. Each ion surrounded by several of the opposite ions and this situation is repeated throughout the structure. Any one sodium ion is surrounded by chlorine ions and each chlorine ion is surrounded by sodium ions.
The same convention is used for the production of all chemical formulas. A balanced molecule must be produced for a chemical compound. Al3+ has to combine with three Cl- ions to give the balanced molecule with the formula AlCl3. This and the previous examples represent the empirical formulas. An empirical formula is the lowest ratio of all of the atoms that are present. It does not necessarily represent the true number of atoms that are in association with each other. The formula that does represent the true numbers of atoms is the molecular formula. The molecular formula shows the number of atoms of each component is a system. In some cases the molecular formula may be the same as the empirical formula, but in others it is impossible to define the molecular formula. Such is the case with ionic compounds and metals which form a crystal lattice.
The chemical formula of an ionic compound is determined by knowing what elements are present, the valency of the elements, and the charge on the ions. The valency of an element is its combining power. In the preceding example the aluminum has a valency of three and the chlorine has a valency of one. These two elements must combine in a ratio of 1:3 to give an electrically balanced compound. The charge of an aluminum ion is +3, and the change of a chlorine ion is -1. So by combining the correct number of the appropriate ions an electrically neutral compound is produced. By knowing the valency and the charges of the elements involved the correct chemical formula can be calculated quite easily.
To calculate the chemical formula of an ionic compound, the correct chemical symbol for each element present is first written down. Next the valency of each ion is specified (this is both the charge and the number associated with that charge). Finally the quantities of the ions present are balanced so that a neutral compound is obtained. With some compounds this is relatively easy. For example, sodium chloride is NaCl, a simple 1:1 mixture. AlCl3 is a 1:3 mixture. It becomes slightly more complicated when the valancies do not readily match as with the ionic compound aluminum oxide. Aluminum has a +3 charge and oxygen has a -2 charge. To balance these requires a ratio of 1:1.5, but by convention only whole numbers are used. To get around this the whole compound is multiplied by two to eliminate the fraction, giving a ratio of 2:3. Thus the formula is Al2O3.
The same rules outlined above also apply when polyatomic ions are considered, i.e. those ions that consist of more than one element.Calcium carbonate consists of Ca which has a +2 charge and carbonate (CO3) which has a -2 charge. So the correct, balanced chemical formula for calcium carbonate is CaCO3. When working out the chemical formula it is convenient to put parentheses around the polyatomic ion to ensure that only the number of polyatomic ions is altered and not the number of the constituent atoms as this would give an entirely different compound. When the final formula is produced, the parentheses can be removed if there is only one example of the polyatomic ion present in the compound. If, however, there are several of the polyatomic ions present, then the parentheses are retained. This logic produces such chemical formulas as NaOH, CuSO4, and Mg(NO3)2.
In covalent compounds there are no ions. Instead the atoms involved share electrons. Covalent compounds lack the crystal lattices common in ionic compounds. This means that the chemical formula is a lot less complex. The true situation, i.e. the molecular formula, can be represented more easily. Organic compounds are covalently bonded. Thus, the chemical formula of an organic compound represents exactly what is present and in the correct ratios.
Once the chemical formulas for molecules are determined, then chemical equations can be worked out. By knowing the correct chemical formula for each component in a reaction, the amount of each reactant and the formula of the reactions products can be determined.
Chemical formulas can also note the appropriate state of matter for each participant in a reaction. A subscript is given after the formula to indicate if it is present as a solid, liquid, gas, or if it is in solution. The subscripts used for this are s, l, g, and aq, respectively. The state of matter may affect the way in which the molecule takes part in a chemical reaction.
The structural formula of a compound is based on the chemical formula. At its most basic the structural formula is the chemical formula showing the bonds that are present. More advanced structural formulas will indicate bond angles and relative placements of the different atoms present.
The chemical formula of a substance is a way of showing what elements are present in a compound and what relationships they have to each other. The chemical formula provides a great deal of information about a substance in shorthand form. In addition, the chemical formula of a substance makes it easier to determine what is happening in chemical reactions and chemical equations.
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