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The Language of Chemistry - 05

Chemical Nomenclature

Chemical nomenclature is the scientific method of naming compounds systematically. In the previous centuries, chemical compounds were being named in an ad-hoc manner, till finally it dawned that a naming system was needed to bring some order into the chaos.

Today, under the guidance of IUPAC (International Union of Pure and Applied Chemistry), chemistry follows a more-or-less systematic naming system. The goal of chemical nomenclature is to identify each chemical compound uniquely through a systematic naming system, and to also convey extra information about the very structure of the compound from the name itself.

Binary Compounds

Binary compounds are made up of two elements.

Binary covalent compounds

Some binary covalent compounds are still identified by names that they have been known by for years, such as water (H₂O) and ammonia (NH₃), but most are now systematically named. Binary covalent compounds consists of two non-metallic elements of the form A_xB_y, where A and B are non-metals, and x and y are subscripts (absence of subscript denotes 1).

There are two tables which are useful for naming purposes.

• Table – table of prefixes used for naming the binary covalent compounds.

Table 3: Roots of names of anions.(Adapted from: Bishop, Introduction Chemistry, p. 98)

Anion	Root for 2nd element
N3−	nitr
P3−	phosph
O2−	ox
S2−	sulph
H−	hydr
F−	fluor
Cl−	chlor
Br−	brom
I−	iod

• Table – table of roots for names of the non-metal elements.

Here's how the naming process works for binary covalent compounds, taking A_xB_y as the format, and N₂O₅ as an example:

1. If x > 1, the associated prefix for the first element from Table is attached to the name of A. (For x = 1, mono is not used at the beginning of the compound's name).
   Thus, the N₂ portion of N₂O₅ will be indicated as dinitrogen.
2. For all y (even if y = 1), select the prefix from Table for the second element. If the B's name begins with a vowel, leave the "a" off the end of the prefixes that end in "a" and "o" off "mono". In this case, B's name (oxygen) does begin with a vowel, and the prefix "penta" does end in "a".
   The name for N₂O₅ now develops to dinitrogen pent−.
3. Using Table , attach the root of the name of the second element B to the developing name.
   The name for N₂O₅ now develops to dinitrogen pentox−.
4. Add −ide to the end of the name.
   The name for N₂O₅ is dinitrogen pentoxide.

Binary hydrogen compounds (non acids)

When naming hydrogen based binary covalent compounds which are not acids, the prefixes need not be used. Thus,

• HCl is hydrogen chloride or hydrogen monochloride
• H₂S is hydrogen sulphide or dihydrogen monosulphide.

Binary ionic compounds

Binary ionic compounds consists of a metal and a non-metal element, and is of the form M_xN_y, where M is a metal (cation) and N is a non-metal (anion), and x and y are subscripts (absence of subscript denotes 1).

The first part of the name is the name of cation (i.e. the metal's name), while the root of the second part is derived from Table (which is very similar to Table ) and then suffixed with −ide.

Thus, we have calcium fluoride for CaF₂ and aluminium oxide for Al₂O₃.

Binary acids (Hydracids)

A binary acid (hydracid) contains hydrogen and another non-metal. The names of hydracids begins with hydro− as the first part, followed by the root of the second element as derived from Table , and ends with −ic acid. However, when the root name begins with a vowel, the first part is hydr−, as in the case of hydriodic acid (HI).

Ternary Compounds

Ternary compounds have three or more elements, one of which is usually oxygen.

Ionic compounds with polyatomic ions

Metal cation – Polyatomic anion

In this case, the first part is the name of the metal (cation), and the second part is the name of the polyatomic anion (see Table ). So, CaCO₃ is calcium carbonate, and Fe2(SO4)₃ is iron(III) sulphate.

Polyatomic cation – non-metal anion

The first part is the name of the polyatomic cation, the second part is named with the help of Table just as in the case for non-metal anions in binary ionic compounds. Thus, NH₄Cl is ammonium chloride.

Polyatomic cation – polyatomic anion

The first part is the name of the polyatomic cation, and the second part is the name of the polyatomic anion. An example is (NH4)2SO₄ which is ammonium sulphate.

Oxoacids (Oxyacids)

Oxoacids, or oxyacids, in addition to hydrogen, contains an oxoanion, which is a polyatomic anion containing oxygen.

A list of oxoanions is given in Table , which is a subset of Table , having only those radicals which have oxygen as a component.

The name of an oxoacid begins with the name of the oxoanion, with the −ite or −ate ending of the oxoanion substituted with −ous acid or −ic acid respectively.

Thus we have oxoacid names like nitric acid, HNO₃, (from nitrate oxoanion NO3−) and sulphurous acid, H₂SO₃, (from sulphite oxoanion SO32−).

Bases (alkalis)

Bases or alkalis are simple to name. The first part is the name of the cation, while the second part is hydroxide. So we have names like sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH).

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