Chemical Nomenclature

Chemical nomenclature refers to a systematic method of naming chemical compounds. Chemical nomenclature is a major portion of the language of chemistry. If you are able to write the formula for a compound, you will be able to predict its name and vice versa. We have two methods of naming compounds, the systematic method or the common name. The systematic method comes from the IUPAC (International Union of Pure and Applied Chemistry), a system developed in 1921 by a group made up of chemists from all over the world. This system has specific rules and has developed an unambiguous method of naming compounds. There are no rules for the common names. H₂O is water, for example.

Goals:

Use the periodic table to predict the charge for an atom.

Write the correct formula for a binary ionic compound.

Write the correct formula for an ionic compound that contains a polyatomic ion.

Correctly name an ionic compound.

(A) Binary ionic compounds

Binary ionic compounds have a completely different set of rules from non-binary ionic compounds and binary molecular compounds and it is imperative that the rules are not confused. A binary ionic compound contains only two elements in ionic form, a metal ion and a nonmetal ion. When the formula for an ionic compound is written, the cation (positive metal ion) is written first followed by the anion (negative non-metal ion). When an ionic compound is named the cation is also named first. The cation is named for the metallic element from which it is obtained. The anion is written and named second. Monatomic anions are named from the root name of the element followed by “-ide”. For example, NaCl is named sodium chloride.

We can frequently determine the charge on the ions just from their location on the periodic table. The elements from Group I have one electron in their outer shell and always ionize to a charge of +1. The elements from Group VII (7) have 7 electrons in their outer shell and gain one electron when they form ions for a charge of -1. Many of the metals have more that one possible charge. These elements are named differently. For example, iron forms both a Fe²⁺ and Fe³⁺ ion. To distinguish between these two ions we use the Stock system. In the Stock system, the charge for cation is written in Roman numerals and put in parenthesis immediately following the name of the cation. Fe³⁺ is iron (III). Pb⁴⁺ is lead (IV). Remember a Roman numeral represents the CHARGE on the cation only.

Group I Group II Group III Group V Group VI GroupVII

Li⁺ Be²⁺ N^3— O^2— F^—

Na⁺ Mg²⁺ Al³⁺ P^3— S^2— Cl^—

K⁺ Ca²⁺ Se^2— Br^—

Rb⁺ Sr²⁺ I^—

Cs⁺ Ba²⁺

From Various groups:

Zn²⁺

Cd²⁺

Ag⁺

Some common metals with more than one possible charge:

Fe²⁺ iron (II) or Fe³⁺ iron (III)

Co²⁺ cobalt (II) or Co³⁺ cobalt (III)

Cu⁺ copper (I) or Cu²⁺ copper (II)

Sn²⁺ tin (II) or Sn⁴⁺ tin (IV)

Pb²⁺ lead (II) or Pb⁴⁺ lead (IV)

Cr²⁺ chromium (II) or Cr³⁺ chromium (III)

Mn²⁺ manganese (II) or Mn⁴⁺ manganese (IV)

Hg₂²⁺ mercury (I) or Hg²⁺ mercury (II)

Ionic compounds are electrically neutral. That is, they have the same number of positive and negative charges.

1. Since sodium is Na⁺ and chloride is Cl^—, the formula is NaCl and the name is sodium chloride.

2. Calcium oxide is formed from Ca²⁺ and O^2— and the formula is CaO.

3. Magnesium bromide is formed from Mg²⁺ and Br^—. There must be two Br^— for every Mg²⁺. The formula is MgBr₂.

4. Tin (IV) sulfide is made from Sn⁴⁺ and S^2—. There must two S^2— for every Sn⁴⁺. The formula is SnS₂.

(B) Ternary or Higher Compounds

Compounds containing three or more elements (usually in the form of polyatomic ions) are very common and follow the same system. The only complication is that the formula, charge and name of the polyatomic ion to determine the formula of the compound must be memorized. There is only one common polyatomic cation, ammonium, NH₄⁺. Many of the polyatomic anions contain oxygen with another element. These oxyanions are named from their non-oxygen element with usually either an “-ite” or “-ate” ending. This ending gives an indication of how many oxygens are present in the ion. This list of polyatomic anions must be memorized.

- 1 charge -1 charge

nitrite NO₂^— permanganate MnO₄^—

nitrate NO₃^— acetate C₂H₃O₂^—

hypochlorite ClO^— hydroxide OH^—

chlorite ClO₂^— hydrogen sulfite HSO₃^—

chlorate ClO₃^— hydrogen sulfate HSO₄^—

perchlorate ClO₄^— hydrogen carbonate HCO₃^—

cyanide CN^—

-2 charge -3 charge

sulfite SO₃^2— phosphate PO₄^3—

carbonate CO₃^2—

sulfate SO₄^2—

chromate CrO₄^2—

dichromate Cr₂O₇^2—

oxalate C₂O₄^2—

The best way to understand the polyatomic anions is to draw Lewis structures for them. (Lewis structure will be covered in chapter 8. For now, you will need to memorize.) The rules for writing the formulas for ternary and higher compounds are similar to binary ionic compounds.

1. Barium sulfate is Ba²⁺ with SO₄^2— . There will be one barium per sulfate and the formula is BaSO₄.

2. Calcium hydroxide is Ca²⁺ with OH^—. There will be two hydroxides with one calcium. The formula is Ca(OH)₂. To make sure that we have two OH^— we must use parenthesis around the OH^—. If we wrote the formula without ( ), it would be CaOH₂. This is 1 Ca, 1 O and 2 H. This does not give us 2 OH^—.

3. Iron (III) oxalate. This is Fe³⁺ with C₂O₄^2—. To balance the charges, we must have 2 Fe³⁺ with 3 C₂O₄^2—. The formula would be Fe₂(C₂O₄)₃.

Likewise we can go from the formula to the name.

1. AgNO₃ is Ag⁺ with NO₃^—. The name is silver nitrate.

2. CrPO₄ is Cr³⁺ with PO₄^3—. We know that the charge on the chromium must be +3 since chromium is 1:1 with phosphate and the charge on phosphate is –3. The name is chromium (III) phosphate. (Remember the Roman numeral represents the CHARGE on the cation.)

3. Hg₂Cl₂ is Hg₂²⁺ with 2 Cl^—. Hg₂²⁺ is mercury (I) and the name is mercury (I) chloride.

Binary molecules.

The rules for naming simple molecules (nonmetal with nonmetal) are different to the rules for ionic compounds. The nonmetals that make up molecules can frequently combine in more ways than a metal with nonmetal. For example sulfur and fluorine can combine to make SF₂, SF₄ and SF₆. Sulfur fluoride would not be an unambiguous name.

Rules:

1. Name the first atom as the name of the element using the prefix if there is more than one.

2. Name the second atom as an ion using the prefix to indicate how many.

Prefixes: 1 – mono, 2 – di, 3 – tri, 4 – tetra, 5 – penta, 6 – hexa, etc.

Examples: Cl₂O is dichloride monoxide. CO is carbon monoxide and CO₂ is carbon dioxide.

See your text for naming acids.

Nomenclature Worksheet

Name:________________________

1. Name the following compounds.

(A) KCl _____________________________________________________

(B) NH₄NO₃ _____________________________________________________

(C) HCl(aq) _____________________________________________________

(D) SF₄ _____________________________________________________

(F) Hg₂Cl₂ _____________________________________________________

(G) NaC₂H₃O₂ _____________________________________________________

(G) CuSO₄ _____________________________________________________

(H) NaOH _____________________________________________________

(I) Cu₃PO₄ _____________________________________________________

(J) Ag₂S _____________________________________________________

(K) Zn(ClO₃)₂ _____________________________________________________

(L) MnO₂ _____________________________________________________

(M) Na₂O₂ _____________________________________________________

(N) CsCN _____________________________________________________

(O) FeCrO₄ _____________________________________________________

(P) H₂S(aq) _____________________________________________________

(Q) NaH₂PO₄ _____________________________________________________

(R) KMnO₄ _____________________________________________________

(S) CaCO₃ _____________________________________________________

(T) HClO(aq) _____________________________________________________

Name:________________________

2. Write the formula for each of the following compounds:

(A) Lithium peroxide ________________________________________________

(B) Sulfur trioxide ________________________________________________

(C) Chlorous acid ________________________________________________

(D) Strontium nitride ________________________________________________

(E) Calcium hydroxide ________________________________________________

(F) Aluminum acetate ________________________________________________

(G) Zinc oxide ________________________________________________

(H) Manganese (II) phosphate______________________________________________

(I) Iron (III) nitrite ________________________________________________

(J) Mercury (II) chloride ________________________________________________

(K) Rubidium bromide ________________________________________________

(L) Barium sulfate ________________________________________________

(M) Chromium (III) dichromate ___________________________________________

(N) Sodium hydrogen carbonate __________________________________________

(O) Aluminum sulfite ________________________________________________

(P) Nitric acid ________________________________________________

(Q) Dinitrogen tetroxide ________________________________________________

(R) Ammonium perchlorate ________________________________________________

(S) Magnesium chlorite ________________________________________________

(T) Cesium hydrogen phosphate _______________________________________