NOMENCLATURE

Three Goals:

1.  Interpret information given by a chemical formula

2.  Write formulas for various compounds

3.  Name compounds given a formula

A chemical formula tells:

-  what type of atoms are present

-  the number of each type of atom present

-  ratio of one atom to another

Remember that the Law of Definite Proportions says that elements combine in definite whole number ratios to form compounds. For any given compound, this ratio is always the same.

Before naming a compound, you must determine which category it fits into:

ionic, molecular, acid/base, or organic.

IONIC COMPOUNDS

-  contains cations and anions.

-  Are solids, includes metals and nonmetals. Metals go first in name and formula.

-  Formula unit is the smallest whole number ratio of cations to anions.

NAMING IONIC COMPOUNDS

1.  Write the name of the cation (metal)

2.  Write the name of the anion (ends in “ide”). If it is a polyatomic ion, use the name given.

3.  If the cation (metal) has more than one charge, designate the correct charge with the roman numeral.

Example: Fe(OH)2 Fe(OH)3

iron (II) hydroxide iron (III) hydroxide

DETERMINING THE CHARGE

A.  Monotomic Ions

An ion with only one atom; Mg+2, Na+1, O-2; Most of the charges can be determined by the atom’s position on the periodic table. It is the atom’s oxidation number. Most transition elements have more than one oxidation number – you just have to memorize them.

B.  Polyatomic Ions

An ion made up of more than one atom; NH4+1, SO4-2, C2H3O2-1; The charges given to polyatomic ions apply to the whole group of atoms. NEVER change the subscripts of a polyatomic ion. If more than one ion is needed, a parenthesis is placed around the ion and the subscript is written outside; Example: Al(OH)2

WRITING IONIC FORMULAS

1.  Write the symbol for the cation, then the symbol for the anion.

2.  Balance the charges (use the crisscross method) by placing subscripts and “( )” for polyatomic ions if needed. Monatomic ions don’t need “( )”. Net charges must be zero. Use ion sheet to get charges.

MULTIPLE OXIDATION NUMBERS

Cations can have more than one oxidation number. Example: Cu+1 and Cu+2. It is important to distinguish which ion is in the compound.

There are two ways used in chemistry:

1. Stock System 2. Classical (Latin)

Cu+1 copper (I) cuprous

Cu+2 copper (II) cupric

Sn+2 tin(II) stannous

Sn+4 tin (IV) stannic

Hg2+2 mercury (I) mercurous

Hg+2 mercury (II) mercuric

Examples: cupric sulfate CuSO4 ferrous oxide FeO

copper (II) sulfate iron (II) oxide

cobaltic chloride CoCl3 mercurous nitrate Hg2(NO3) 2

cobalt (III) chloride mercury (I) nitrate

*MEMORIZE**MEMORIZE**MEMORIZE*

NAMING MOLECULAR COMPOUNDS

Rules: Happens between two nonmetals. Use prefixes to identify the number of atoms present. If only one of the first atom (nonmetal), then NO prefix is used. Last element ends in “ide”.

Prefixes:

1  - mono 5 - penta 9 - nona

2  - di 6 - hexa 10 - deca

3  - tri 7 - hepta

4  - tetra 8 - octa

Examples:

CO2 carbon dioxide PCl3 phosphorus trichloride

N2O dinitrogen monoxide P2O3 diphosphorus trioxide

COMMON NAMES

Dihydrogen monoxide, H2O, WATER Nitrogen trihydride, NH3, AMMONIA

WRITING MOLECULAR FORMULAS

To write a molecular formula, you look at the first name. Write the element symbol followed by the number the prefix indicates. If there is no prefix, it is understood to be one. Then write the second element symbol followed by the number the prefix indicates.

Example: silicon dioxide becomes Si (one) O (di) or SiO2

Diphosphorus pentoxide becomes P (di) O (penta) or P2O5

NAMING ACIDS

Acids are substances which when put in water disassociate to give H+ ions. If you see a compound that starts with an H, chances are it is an acid.


H X à à H+ + X- HCl à à H+ + Cl-

generic Hydrochloric acid

RULES FOR NAMING ACIDS

1.  If the suffix of the anion is “-ide”, the acid compound starts with “hydro-“ and ends with “-ic”. Example: HBr – ion is bromide – hydrobromic acid

2.  If the suffix of the anion is “ate”, the acid compound ends in “ic” acid.

Example: H2CO3 – ion is carbonate – carbonic acid

3.  If the suffix of the anion is “ite”, the acid compound ends in “ous” acid.

Example: HNO2 – ion is nitrite – nitrous acid

WRITING ACID FORMULAS

1.  Determine the ion ending by the acid type – see rules above. So chromic acid means that the anion ends in –ate and the ion must be chromate.

2.  Write H+ and the anion formula. DO the criss cross and determine the formula.

Example: chromic acid comes from Chromate, CrO4-2. So H+ and CrO4-2 become H2CrO4.