Chemical formulas are shorthand ways to represent the number and type of atoms in a compound or molecule, such as H2O for water or NaCl for sodium chloride, or salt. There are several rules to follow when writing chemical formulas, so the process can be rather complex. The more you familiarize yourself with the periodic table and the names of common compounds, the easier it will be to learn how to write chemical formulas.
Use the Periodic Table
To write chemical formulas, acquaint yourself with chemical symbols, most easily found on the periodic table of elements. The periodic table is a chart of all the known elements, and it often includes both the full name of each element and its symbol, such as H for hydrogen or Cl for chlorine. Some of these symbols are obvious, such as O for oxygen, while others are not quite as intuitive with their English name; Na, for example, stands for sodium, but the symbol derives from natrium, the Latin word for sodium. You can use a periodic table to reference the symbols you can’t memorize.
Identifying Chemical Symbols
Before you can write your chemical formula, you need to write down the symbol of each atom present in your molecule or compound. You might be given a name of a compound, such as sodium chloride, and you must determine which atoms are present. Write Na for sodium and Cl for chloride, a form of the element chlorine, which combined create the formula NaCl for sodium chloride, or salt. Covalent compounds created from two nonmetals are easy to write from their name. Prefixes might be present to indicate more than one atom. For example, carbon dioxide’s formula is CO2 because di specifies two oxygen atoms.
Determining the Valence
Ionic compounds, created from a metal and a nonmetal, are more complex than covalent compounds because they involve charged atoms. You might notice that some periodic tables list valences, or a positive or negative charge. Cations, or positive ions, are found in group 1, with a +1 charge; group 2, with a +2 charge; and the transition elements, found in groups 3 through 12. Groups 13, 14 and 18 have variable charges, and groups 15 through 17 are anions, meaning they have negative charges.
Balancing the Charges
Finding the valence of each element is essential when writing, because you need to balance your chemical formula, so it has no charge. For example, write the symbols for magnesium oxide along with their respective charges. Magnesium, or Mg, has a +2 charge, and oxide, which refers to oxygen, has a -2 charge. Since the sum of +2 and -2 is O, you end up with only one atom each of magnesium and oxygen. Combine the symbols to form MgO, the formula for magnesium oxide.
Writing the Chemical Formula
Chemical formulas use subscripts to tell how many of each atom are present in a molecule or compound. In the previous example, you would write MgO because there is only one atom of each element; notice you do not use the subscript 1 for only one atom. On the other hand, to balance magnesium chloride, written MgCl2, you need two chlorine atoms per one magnesium atom; the 2 is written as a subscript next to Cl to indicate two chlorine atoms.
As you practice writing chemical formulas, you will become familiar with chemical nomenclature, or terms used to describe compounds. Elements ending in -ide, for example, can be found in groups 15 through 17 on the periodic table. Roman numerals in parentheses, as seen in iron(II), denote charges, a +2 in this case. When polyatomic ions, or groups of atoms like hydroxide, written OH, are combined in a compound, they are put in parentheses in chemical formulas, as seen in Al(OH)3, the formula for aluminum hydroxide.
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