Atoms, Molecules and Ions

Naming inorganic compounds


Naming Inorganic Compounds

With over 10 million known chemicals, and potentially dangerous results if chemicals are combined in an incorrect manner, imagine the problem if you are in the lab and say "mix 10 grams of that stuff in with this stuff". We need to be very clear on identification of chemicals.

Two early classifications of chemical compounds:

  1. Organic compounds. These contain the element Carbon (C). "Life on earth is carbon based"
  2. Inorganic compounds. All other compounds

Organic compounds were associated with living organisms, however, a large number of organic compounds have been synthesized which do not occur in nature, so this distinction is no longer valid.

Ionic compounds: (an association of a cation and an anion)

The positive ion (cation) is always named first and listed first in writing the formula for the compound.

The vast majority of monatomic (composed of a single atom) cations are formed from metallic elements:

If an element can form more than one positive ion, the positive charge of the ion is indicated by a Roman numeral in parentheses following the name of the metal:

Iron and copper are examples of transition metals. They occur in the block of elements from IIIB to IIB of the periodic table.

The transition metals often form two or more different monoatomic cations.

click on picture for larger image

 

An older nomenclature for distinguishing between the different ions of a metal is to use the suffixes -ous and -ic. The suffix -ic will indicate the ion of higher ionic charge:

Note that the different ions of the same element often have quite different chemical properties (again, pointing to the importance of electrons in determining chemical reactivity).

Ionic compounds: Anions

Monatomic anions are usually formed from non-metallic elements. They are named by dropping the ending of the element name and adding -ide:

Some common polyatomic anions include:

Many polyatomic anions contain oxygen, and are referred to as oxyanions. When an element can form two different oxyanions the name of the one that contains more oxygen ends in -ate, the one with less ends in -ite:

Note that unlike the -ous and -ic suffix nomenclature to distinguish the different cations of a metal, the -ite and -ate suffix is used to distinguish the relative amounts of the oxygen atoms in a (polyatomic) oxyanion (in the above examples the ionic charge is the same for the -ite and -ate ions of a specific oxyanion).

Now to get really perverse....

Some compounds can have multiple oxyanion forms (the oxyanions involving the halogens, for example):

Note again, that the number of Oxygens relative to the Chlorine is changing, but that the ionic charge is not.

How do we name these? The -ite and -ate suffixes are still used, but we have to add an additional modification to allow us to distinguish between the four forms:

It should be pointed out that some of the naming of ions is historical and is not necessarily systematic. It may be frustrating and confusing, but its all part of chemistry's rich history.

Many polyatomic anions that have high (negative) charges can add one or more hydrogen cations (H+) to form anions of lower effective charge. The naming of these anions reflects whether the H+ addition involves one or more hydrogen ions:

Acids

 

Cl- chloride anion

HCl hydrochloric acid 

S2- sulfide anion

H2S hydrosulfuric acid 

Using the -ic suffix here may seem a bit inconsistent since it was used in naming metal cations to indicate the form which had the higher positive charge. However, when you think about it, the acid compound has a higher net positive charge than the anion from which it is derived (the anion is negatively charge and the associated acid is neutral).

Again, things get complicated when we consider the acids of oxyanions:

 

ClO- hypochlorite ion 

HClO hypochlorous acid 

ClO2- chlorite ion 

HClO2 chlorous acid 

ClO3- chlorate ion 

HClO3 chloric acid 

ClO4- perchlorate ion 

HClO4 perchloric acid 

This is confusing: we previously had used the -ous and -ic suffixes to indicate the ionic charge differences in metal cations (-ic had a higher positive charge). Although in comparison to the ionic form, the -ic and -ous acid forms have a higher net positive charge, the -ic suffix would indicate forms with a higher oxygen content, and not an apparent charge difference.

Molecular compounds

Although they may not be ionic compounds, chemically bonded compounds of two different elements can be thought of as being made up of an element with a more positive chemical nature, and one that has a more negative nature in comparison. Elements on the left hand side of the periodic table prefer to donate electrons (thus taking on a more positive chemical nature), and elements on the right hand side prefer to accept electrons (thus taking on a more negative chemical nature). The element with the more positive nature in a compound is named first. The second element is named with an -ide ending.

Often a pair of elements can form several different molecular compounds. For example, Carbon and Oxygen can form CO and CO2. Prefixes are used to identify the relative number of atoms in such compounds:

Such prefixes can extend for quite a way for some organic and polymeric compounds (a common detergent in shampoos is sodium dodecylsulfate, or "SDS", also known as Sodium Laurel Sulfate because it sounds more benign and holistic). The list of such prefixes includes:
 

Prefix

Meaning

Mono-

1

Di-

2

Tri-

3

Tetra-

4

Penta-

5

Hexa-

6

Hepta-

7

Octa-

8

Nona-

9

Deca-

10

Undeca-

11

Dodeca-

12


1996 Michael Blaber