CHEM101: General Chemistry I

1 Naming the Binary Molecules

The system used for naming chemical substances depends on the nature of the molecular units making up the compound. These are usually either ions or molecules; different rules apply to each. In this section, we discuss the simplest binary (two-atom) molecules.

Numbers in Names

It is often necessary to distinguish between compounds in which the same elements are present in different proportions; carbon monoxide CO and carbon dioxide CO₂ are familiar to everyone. Chemists, perhaps hoping it will legitimize them as scholars, employ Greek (of sometimes Latin) prefixes to designate numbers within names; you will encounter these frequently, and you should know them:

You will occasionally see names such as dihydrogen and dichlorine used to distinguish the common forms of these elements (H₂, Cl₂) from the atoms that have the same name when it is required for clarity.

Examples:

• N₂O₄ - dinitrogen tetroxide (note the missing a preceding the vowel)
• N₂O - dinitrogen oxide (more commonly, nitrous oxide)
• SF₆ - sulfur hexafluoride
• P₄S₃ - tetraphosphorus trisulfide (more commonly, phosphorus sesquisulfide)
• Na₂HPO₄ - disodium hydrogen phosphate
• H₂S - hydrogen sulfide (we skip both the di and mono)
• CO - carbon monoxide (mono- to distinguish it from the dioxide)
• CaSO₄·½H₂O - calcium sulfate hemihydrate (In this solid, two CaSO₄ units share one water of hydration between them; more commonly called Plaster of Paris)

It will be apparent from these examples that chemists are in the habit of taking a few liberties in applying the strict numeric prefixes to the more commonly known substances.

Binary Compounds of Non-Metals

These two-element compounds are usually quite easy to name because most of them follow the systematic rule of adding the suffix -ide to the root name of the second element, which is normally the more negative one. Several such examples are shown above.

But as noted above, there are some important exceptions in which common or qH₂O (water, not dihydrogen oxide)/

• H₂O₂ (hydrogen peroxide, not dihydrogen dioxide)
• H₂S (hydrogen sulfide, not dihydrogen sulfide)
• NH₃ (ammonia, not nitrogen trihydride)
• NO (nitric oxide, not nitrogen monoxide)
• N₂O (nitrous oxide, not dinitrogen oxide)
• CH₄ (methane, not carbon tetrahydride)

2 Naming the Chemical Ions

An ion is an electrically charged atom or molecule – that is, one in which the number of electrons differs from the number of nuclear protons. Many simple compounds can be regarded, at least in a formal way, as being made up of a pair of ions having opposite charge signs.

Cations

The positive ions, also known as cations, are mostly those of metallic elements which simply take the name of the element itself.

The only important non-metallic cations you need to know about are

(Later on, when you study acids and bases, you will learn that the first two represent the same chemical species.)

Multivalent Cations

Some of the metallic ions are multivalent, meaning that they can exhibit more than one electric charge. For these there are systematic names that use Roman numerals, and the much older and less cumbersome common names that mostly employ the Latin names of the elements, using the endings -ous and -ic to denote the lower and higher charges, respectively. (In cases where more than two charge values are possible, the systematic names are used.) The only ones you need to know in this course are the following:

* The mercurous ion is a unique double cation that is sometimes incorrectly represented as Hg⁺.

Anions

The non-metallic elements generally form negative ions (anions). The names of the monatomic anions all end with the -ide suffix:

There are a number of important polyatomic anions which, for naming purposes, can be divided into several categories. A few follow the pattern for the monatomic anions:

Oxyanions

The most common oxygen-containing anions (oxyanions) have names ending in -ate, but if a variant containing a small number of oxygen atoms exists, it takes the suffix -ite.

The above ions (with the exception of nitrate) can also combine with H+ to produce acid forms having smaller negative charges. For rather obscure historic reasons, some of them have common names that begin with -bi which, although officially discouraged, are still in wide use:

Chlorine, and to a smaller extent bromine and iodine, form a more extensive series of oxyanions that requires a somewhat more intricate naming convention:

3 Names of Ion-Derived Compounds

These compounds are formally derived from positive ions (cations) and negative ions (anions) in a ratio that gives an electrically neutral unit.

Salts

Salts, of which ordinary salt (sodium chloride) is the most common example, are all solids under ordinary conditions. A small number of these (such as NaCl) do retain their component ions and are properly called ionic solids. In many cases, however, the ions lose their electrically charged character and form largely-non-ionic solids such as CuCl₂. The term ion-derived solids encompasses both of these classes of compounds.

Most of the cations and anions described above can combine to form solid compounds that are usually known as salts. The one overriding requirement is that the resulting compound must be electrically neutral: thus the ions Ca²⁺ and Br^– combine only in a 1:2 ratio to form calcium bromide, CaBr₂. Because no other simplest formula is possible, there is no need to name it calcium dibromide.

Since some metallic elements form cations having different positive charges, the names of ionic compounds derived from these elements must contain some indication of the cation charge. The older method uses the suffixes -ous and -ic to denote the lower and higher charges, respectively. In the cases of iron and copper, the Latin names of the elements are used: ferrous, cupric.

This system is still widely used, although it has been officially supplanted by the more precise, if slightly cumbersome Stock system in which one indicates the cationic charge (actually, the oxidation number) by means of Roman numerals following the symbol for the cation. In both systems, the name of the anion ends in -ide.

Acids

Most acids can be regarded as a combination of a hydrogen ion H⁺ with an anion; the name of the anion is reflected in the name of the acid. Notice, in the case of the oxyacids, how the anion suffixes -ate and -ite become -ic and -ous, respectively, in the acid name. Yes, chemistry has a grammar much like that of any other language – and quite a lot of it is irregular!

Source: Stephen Lower, http://www.chem1.com/acad/webtext/intro/int-5.html#SEC3
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