|Common Names||Ionic Compounds||Positive Ions||Negative Ions|
|Polyatomic Negative Ions||Naming Polyatomic Ions||Covalent Compounds||Acids|
|Metals with Non-metals||Non-metals with Non-metals|
Long before chemists knew the formulas for chemical compounds, they developed a system of nomenclature that gave each compound a unique name. Today we often use chemical formulas, such as NaCl, C12H22O11, and Co(NH3)6(ClO4)3, to describe chemical compounds. But we still need unique names that unambiguously identify each compound.
Some compounds have been known for so long that a systematic nomenclature cannot compete with well-established common names. Examples of compounds for which common names are used include water (H2O), ammonia (NH3), and methane (CH4).
The names of ionic compounds are written by listing the name of the positive ion followed by the name of the negative ion.
We therefore need a series of rules that allow us to unambiguously name positive and negative ions before we can name the salts these ions form.
Monatomic positive ions have the name of the element from which they are formed.
Some metals form positive ions in more than one oxidation state. One of the earliest methods of distinguishing between these ions used the suffixes -ous and -ic added to the Latin name of the element to represent the lower and higher oxidation states, respectively.
Chemists now use a simpler method, in which the charge on the ion is indicated by a Roman numeral in parentheses immediately after the name of the element.
Polyatomic positive ions often have common names ending with the suffix -onium.
Negative ions that consist of a single atom are named by adding the suffix -ide to the stem of the name of the element.
|Practice Problem 4
Predict the formula of the compound that forms when magnesium metal reacts with nitrogen to form magnesium nitride.
|HCO3-||bicarbonate||HSO4-||hydrogen sulfate (bisulfate)|
At first glance, the nomenclature of the polyatomic negative ions in the table above seems hopeless. There are several general rules, however, that can bring some order out of this apparent chaos.
The name of the ion usually ends in either -ite or -ate. The -ite ending indicates a low oxidation state. Thus,the NO2- ion is the nitrite ion.
The -ate ending indicates a high oxidation state. The NO3- ion, for example, is the nitrate ion.
The prefix hypo- is used to indicate the very lowest oxidation state. The ClO- ion, for example, is the hypochlorite ion.
The prefix per- (as in hyper-) is used to indicate the very highest oxidation state. The ClO4- ion is therefore the perchlorate ion.
There are only a handful of exceptions to these generalizations. The names of the hydroxide (OH-), cyanide (CN-), and peroxide (O22-) ions, for example, have the -ide ending because they were once thought to be monatomic ions.
|Practice Problem 5
The bone and tooth enamel in your body contain ionic compounds such as calcium phosphate and hydroxyapatite. Predict the formula of calcium phosphate, which contains Ca2+ and PO43- ions. Calculate the value of x, if the formula of hydroxyapatite is Cax(PO4)3(OH).
Oxidation states also play an important role in naming simple covalent compounds. The name of the atom in the positive oxidation state is listed first. The suffix -ide is then added to the stem of the name of the atom in the negative oxidation state.
As a rule, chemists write formulas in which the element in the positive oxidation state is written first, followed by the element(s) with negative oxidation numbers.
The number of atoms of an element in simple covalent compounds is indicated by adding one of the following Greek prefixes to the name of the element.
|1 mono-||6 hexa-|
|2 di-||7 hepta-|
|3 tri-||8 octa-|
|4 tetra-||9 nona-|
|5 penta-||10 deca-|
The prefix mono- is seldom used because it is redundant. The principal exception to this rule is carbon monoxide (CO).
Simple covalent compounds that contain hydrogen, such as HCl, HBr, and HCN, often dissolve in water to produce acids. These solutions are named by adding the prefix hydro- to the name of the compound and then replacing the suffix -ide with -ic. For example, hydrogen chloride (HCl) dissolves in water to form hydrochloric acid; hydrogen bromide (HBr) forms hydrobromic acid; and hydrogen cyanide (HCN) forms hydrocyanic acid.
Many of the oxygen-rich polyatomic negative ions in Table 2.1 form acids that are named by replacing the suffix -ate with -ic and the suffix -ite with -ous.
|Acids containing ions ending with ide often become||hydro -ic acid|
|Acids containing ions ending with ate usually become||-ic acid|
|Acids containing ions ending with ite usually become||-ous acid|
|SO32-||sulfite||H2SO 3||sulfurous acid|
Complex acids can be named by indicating the presence of an acidic hydrogen as follows.
|NaHCO3||sodium hydrogen carbonate (also known as sodium bicarbonate)|
|NaHSO3||sodium hydrogen sulfite (also known as sodium bisulfite)|
|KH2PO4||potassium dihydrogen phosphate|