Rocky Star Chemistry

Chemical Nomenclature

The trivial name is not entirely satisfactory for a name that will identify precisely the composition of a compound. Therefore it is necessary to have a scientific systematic method of identifying compounds by name. This system considers the compound to be composed of two parts, one positive and one negative. The positive part (positive ion or least electronegative element) is named and written first. The negative part, generally nonmetallic, follows. The names are modified with suffixes and prefixes to identify the different types or classes of compounds. Thus, the compound composed of sodium ions and chloride ions is named sodium chloride; the compound composed of iron(II) ions and chloride ions is named iron(II) chloride (read as "iron-two chloride"); the covalent compound composed of carbon and oxygen atoms in a 1:2 ratio is named carbon dioxide.

There are two general rules that are very helpful in naming the different types of compounds:

Rule 1: The usual oxidation numbers of representative elements of Groups I, II, and III elements are +1, +2, and +3 respectively. That is for these elements the oxidation number is the same as the group number.

Rule 2: The oxidation number of elements in the other periodic groups are variable. However, in practically all compounds, combined oxygen has an oxidation number of -2; combined hydrogen has an oxidation number of +1; and the halogens (Group VII) have an oxidation number of -1 in binary compounds with metals.

1. Binary Compounds:

Binary compounds contain two different elements, and their names have two parts: The name of the more positive element followed by the name of the more negative element modified to end in ide (see part "D" for exceptions)

A. Binary compounds in which the positive element has a fixed oxidation number:

Most of these compounds contain a metal and a nonmetal. The chemical name is composed of the name of the metal followed by the name of the nonmetal, which has been modified to an identifying stem plus the suffix ide.

For example: Sodium chloride, NaCl, is composed of one atom each of sodium and chlorine. The name of the metal is written first then the second part is derived from the nonmetal, chlorine, by using the stem chlor and adding the ending ide and is named chloride. Binary names of the more common negative ions are found in your text under "Common Ions."

B. Binary compounds containing metals of varying oxidation numbers:

The official system is designated by the International Union of Pure and Applied Chemistry (IUPAC). In this system, when a compound contains a metal that can have more than one oxidation state, the oxidation number of the metal is designated by a Roman numeral placed in parentheses immediately following the name of the metal. The negative element is treated as in part "A" above.

When a metal has only one possible oxidation state, it is not necessary to distinguish one oxidation state from another, therefore Roman numerals are not needed. Thus, CaCl₂ is read as calcium chloride and NOT calcium(II) chloride, since the oxidation number is understood to be +2.

In classical nomenclature, when the metallic ion has only two oxidation numbers, the name of the metal (usually the Latin name) is modified with the suffixes ous and ic to distinguish between the two. The lower oxidation state is given the ous ending, and the higher one the ic ending.

Examples	FeCl2	Ferrous chloride	Fe+2	lower oxidation state
	FeCl3	Ferric chloride	Fe+3	higher oxidation state
	CuCl	Cuprous chloride	Cu+	lower oxidation state
	CuCl2	Cupric chloride	Cu+2	higher oxidation state

The common metals that have more than one oxidation number are listed in your text under "Common Ions." Notice that the classical endings do not give the actual oxidation number of the metal but merely indicates that at least two oxidation states exist.

C. Binary compounds containing two nonmetals:

The chemical bond that exists between two nonmetals is predominantly covalent. In a covalent compound, positive and negative oxidation numbers are assigned to the elements according to their electronegativities. The least electronegative element is named first. The common nonmetals arranged in their order of increasing electronegativity form the series:

Si, B, P, C, N, H, B, I, Br, Cl, O, F

In a compound between two of these elements, the element that occurs first in the series is written first and named first. The name of the second element retains the modified binary ending. A Latin or Greek prefix (mono, di, etc.) is attached to the name of each element to indicate the number of atoms of that element in the molecule. The prefix mono is usually omitted except when needed to differentiate between two or more compounds, such as carbon monoxide, CO, and carbon dioxide, CO₂. Some common prefixes and their numerical equivalences are:

Mono = 1

Di = 2

Tri = 3

Tetra = 4

Penta = 5

Hexa = 6

Hepta = 7

Octa = 8

Nona = 9

Deca = 10

Examples	CO	Carbon monoxide	N2O	Dinitrogen monoxide
	CO2	Carbon dioxide	N2O4	Dinitrogen tetroxide
	PCl3	Phosphorous trichloride	NO	Nitrogen oxide
	PCl5	Phosphorous pentachloride	N2O3	Dinitrogen trioxide
	P2O5	Diphosphorous pentoxide	S2Cl2	Disulfur bichloride
	CCl4	Carbon tetrachloride	S2F10	Disulfur decafluoride

Some special names such as ammonia (NH₃) and ammonium (NH⁺⁴) are exceptions to the system. Most of these will be found in your text under "Common Ions."

D. Exceptions that use ide endings:

Three notable exceptions that use the ide ending are hydroxide (OH^-), cyanides (CN^-), and ammonium (NH⁺⁴) compounds. These polyatomic ions, when combined with another element, take the ide ending, even though more than two elements are present in the compound.

Examples	NH4I	Ammonium iodide
	Ca(OH)2	Calcium hydroxide
	KCN	Potassium cyanide

E. Acids derived from binary compounds:

Certain binary hydrogen compounds, when dissolved in water, form solutions that have acid properties. Because of this, they are given acid names in addition to their binary (ide) names. To name a binary acid, place the prefix hydro in front of, and the suffix ic after, the stem of the nonmetal name. Then add the word acid

Examples	HCl	Hydro chlor ic acid
	H2S	Hydro sulfuric acid

F. Salts of binary aids:

Salts are ionic compounds of anions (negative ions) and cations (positive ions). When the hydrogen of an acid is replaced by a metal ion or an ammonium ion, the compound formed is a salt. Salts are named by combining the names of the cation and the anion, the name of the cation being given first. Thus, when the hydrogen is replaced in hydrochloric (HCl) acid by the metal sodium, the salt is named sodium chloride.

2. Ternary oxy-acids:

A. Inorganic ternary compounds containing hydrogen, oxygen, and one other element are called oxy-acids:

The other element is usually a nonmetal, but in some cases it can be a metal. The ous-ic system is used in naming ternary acids. The ous ending is used to indicate the lower oxidation number of the other element and the ic ending the higher number. To name these acids, we place the ending, ous-ic after the stem of. the element other than hydrogen or oxygen and add the word acid. If the element has only one oxidation number, the ending ic is used.

B. Salts of ternary acids:

As with binary acids, a salt is formed when the hydrogen is replaced by a metal or ammonium ion. These salts are named in the same way as binary salts.

Examples	Ternary oxy-acid	Ternary oxy-salt
	ous ending of acid becomes	ite ending in salt
	ic ending of acid becomes	ate ending in salt

Thus the sulfite ion (SO₃⁺²) is derived from sulfurous acid (H₂SO₃) and the sulfate ion (SO₄⁺²) from sulfuric acid (H₂S0₄). Therefore Na₂SO₃ is named sodium sulfite and Na₂SO₄ is named sodium sulfate.

C. Salts with more than one kind of positive ion:

Salts can be formed from acids which contain two or more hydrogen atoms by replacing only one of the hydrogen atoms with a metal or by replacing both hydrogen atoms with different metals. Each group is named first, and then the appropriate salt ending is added.

Examples	with 2 H = NaH2PO4	Sodium dihydrogen phosphate
Using phosphoric acid - H3PO4	with 1 H = Na2HPO4	Disodium hydrogen phosphate
	with 0 H = Na3PO4	Trisodium phosphate (TSP)

D. Bases:

Inorganic bases contain the hydroxide ion (OH^-), in chemical combination with a metal ion or ammonium. The name of the positive ion is given and the word hydroxide is added.

Examples	NaOH	Sodium hydroxide
	KOH	Potassium hydroxide
	NH4OH	Ammonium hydroxide

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