109

Unit 1
Elements of Chemical Structure and Inorganic Nomenclature

1-9. NAMING ANIONS

There are generally two types of anions. Many anions are elemental; that is they are made of only one atom of one element. Others are composed of groups of atoms of one or more elements that pass through a reaction unchanged in most cases. This latter group of anions is called radicals. We will concern ourselves first with the naming of elemental or monatomic anio ns.

The names of the elemental anions are made by adding the -ide suffix to the root of the element's name. Thus anions formed by chlorine (Cl-1) are called chloride ion; anions formed by oxygen (O-2) are called oxide ion.

Examples

The most common type of anionic radicals consists of a central atom covalently bonded to a number of atoms of oxygen. Monovalent anionic radicals (Valence = -1) normally contain three oxygen atoms; radicals with negative valences greater than one normally contain four oxygen atoms. The names for these normal types of radicals are formed from the root for the name of the central atom plus the suffix -ate. Thus, ClO3 -1 is named chlorate and SO4 -2 is named sulfate. It is important to note that these generalizations have exceptions. The best way to remember the names and formulas for the radicals is to memorize the common ones. Most of these are listed in this e-book.

Sometimes a central atom may be bonded to a different number of oxygen atoms than normal; in other words, a series of radicals may be formed with the same central atom. Different suffixes and prefixes are used to name these different radicals. When there is one less oxygen atom than normal, the suffix -ite is used. The name for ClO2 -1 is chlorite; SO3 -2 is called sulfite.

Occasionally, there are other radicals in a series. This is especially true of the halides (fluoride, chloride, bromide, and iodide ions). If there are two less oxygen atoms than usual, the -ite suffix is used with the prefix hypo-. For example, ClO-1 is called hypochlorite. If there is one more oxygen atom than normal, the -ate suffix is used in combination with the prefix per-, so ClO4 -1 is named perchlorate.

A chart summarizing the use of the prefixes and the series of radicals formed by chlorine as examples follows:

PREFIX SUFFIX NAME OF ION RADICAL
hypo- --ite hypochlorite ClO-1
  --ite chlorite ClO2-1
  --ate chlorate ClO3-1
per- --ate perchlorate ClO4-1

Examples

There are several significant exceptions to the rules for the naming of anionic radicals. The most important is the previously mentioned carbonate radical (CO3-2). Several others bear mentioning because you are likely to see them in medicine. .                                                                                                                                          

  1. Certain radicals are derived when hydrogen is removed from an acid to form a charge group of atoms (radical). If one hydrogen is removed, the radical gets the prefix bi. This indicates that one hydrogen is missing.

Example 1: When carbonic acid (H2CO3) gives up one hydrogen on, it loses a positively charged hydrogen atom. It becomes a radical HCO3 and is assigned the name bicarbonate. Bi indicates one hydrogen was removed.

Example 2: (H2PO4 -1 is called the bi phosphate radical because it was derived from phosphoric acid (H3PO4) by removing one hydrogen atom.

  1. Several radicals do not follow any of the above rules. Their names and formulas must be memorized. Some of the most common are hydroxide (OH -1), peroxide (O2 ), and thiosulfate (S2O3-2 ).
  1. Occasionally, metals with valences higher than +1 will form salts that contain oxide or hydroxide ion. When these occur in the middle of the formula, they are referred to as either oxy- or hydroxy-, respectively. Number prefixes are used to denote the number of them.