Rules for writing Chemical Formulae of Compounds

Rules for writing Chemical Formulae of Compounds

To write chemical formulae of compounds, following steps are involved:

1. Every compound is supposed to consist of two types of radicals namely cation and anion. First of all name of types of ions or radicals are identified in the given compounds. Then symbols or formulae of both ions or radicals are written side by side (showing cation first then anion) with their real or formal charges (oxidation numbers) or valencies in superscripts (at the top or at the upper side each ion or radical).

2. Now the correct combination of ions is used to produce a compound with a net charge of zero. To do so, the real or formal charges or valencies of corresponding cation and anion are cross multiplied. If the charges are same then they are cancelled out otherwise they become the subscripts of ions or radicals showing the ratios of atoms. Don't write the subscript '1'. Parentheses surrounding the polyatomic ion and a subscript will be used when there is more than one polyatomic ion in the formula.

Solved Examples

Example # 1. Write down chemical formula of calcium oxide.

Example # 2 Write the correct formula for Barium Fluoride.

Example # 3 Write the proper formula for the ionic compound lithium bromide.

Example # 4; Show the correct formula for Zinc Nitrate.

Example # 5; Show the correct formula for ferric sulphate.

Naming Binary Ionic Compounds

A binary ionic compound is mostly a salt consisting of only two elements in which both elements are mostly monoatomic ions, a cation and an anion. Binary compounds of a metal and a nonmetal contain a metallic cation and a nonmetallic anion. When naming these compounds, its composition must be considered.

1. The cation (which is metallic) is listed first and the anion (which is generally non-metallic) second.

2. The cation takes the name of its elemental form. 

For example, Li⁺ would be called "Lithium".

3. The anion name uses the first part of its elemental name or root name, and subsequently adding the suffix "-ide" i.e. for the non-metal (the second element in the formula) write the name as it appears on the Periodic Table and then replace the ending with ide. 

For example, 

Br^– would be called "Bromide", 

S^2– would be called sulphide and 

O^2– would be called oxide.

Recall that the alkali metals form only ions with a +1 charge, the alkaline earth metals form only ions with a +2 charge, and aluminum forms only the ion Al³⁺. For these ions, the name of the element followed by the term ion is an unambiguous name. For example, the sodium ion can only be Na⁺, the calcium ion only Ca²⁺.

According to IUPAC rules, the names of all other metallic cations contain the name of the element followed by its oxidation state (in parentheses) in that ion. This rule prevents ambiguity. The name chromium ion does not say whether the ion is Cr²⁺ or Cr³⁺; the proper names for these ions are chromium(II) and chromium(III) respectively.

Before leaving this group of compounds, we should mention again the second and less-preferred method of naming cations of the same element in different oxidation states. This older method gives the ending ous to the ion of lower oxidation state and the ending ic to the ion of higher oxidation state. Often this system also uses the Latin root of the name of the element. Thus, in this system, Fe²⁺ is ferrous and Fe³⁺ is ferric; Pb²⁺ is plumbous and Pb⁴⁺ is plumbic.

Examples

Naming Binary Molecular compounds containing two non-metals but not hydrogen

Binary compounds of two non-metals, neither of which is hydrogen, are molecular rather than ionic. They do not contain cations and anions. Carbon dioxide (CO₂) and phosphorus trichloride (PCl₃) are examples of such compounds. They are named using prefixes like di, tri, tetra, penta, hexa etc. to state how many atoms of an element are in one molecule of the compound.

The name of the second element is modified to the root of its name followed by the ending ide. In both the formula and the name of these compounds, the most non-metallic element comes first. The prefix mono is often omitted for the first element but never omitted for the second. Thus,

CO is carbon monoxide
SF₆ is sulfur hexafluoride
N₂O is dinitrogen monoxide

3. Binary Acids

The binary compound formed when a halogen or any element, except oxygen (from group VIA of the periodic table), combines with hydrogen can be named as were the binary non-metallic compounds.

 

However, when these compounds are dissolved in water, the solution contains hydrogen ions. Because this property identifies an acid, these compounds must also be named as acids. Therefore, these compounds have two sets of names, one for the pure state and one for the compound dissolved in water.

Two points should be noted:

(1) The acid name has the prefix hydro and the suffix ic. For binary acids, add the prefix –hydro to the root name of the non-metallic anion and change the suffix of the anion name from –ide to –ic. e.g.

      The stem for chlorine is chlor. 

      Cl^– …… chloride ion,  

      HCl ….. hydrochloric acid

(2)  These formulas are always written with hydrogen first. Other non-metals like C, Si, N, P, As etc. form compounds with hydrogen, but they are not acids; their formulas are written with hydrogen last. Methane, CH₄, ammonia, NH₃, and arsine, AsH₃, are some examples.

 

Naming Pseudo-Binary Compounds

Several polyatomic ions act so much like monatomic ions that they are classified as such. These ions are called pseudo-binary ions. They include the ammonium ion, NH₄⁺, the hydroxide ion, OH^-, the cyanide ion, CN^-, and others. Compounds containing these ions are pseudo-binary compounds.

 

The properties of the ammonium ion are much like those of the alkali-metal ions. Compounds containing the hydroxide ion are bases. A general definition of a base is that its aqueous solution contains more hydroxide than hydrogen ions. The cyanide ion behaves very much like a halogen ion. Many compounds containing the cyanide ion are extremely toxic.

NaOH = Sodium hydroxide

KCN=Potassium cyanide

NH₄Cl=Ammonium chloride

If a compound contains two or more different electropositive elements, it is also named as a binary compound. Suppose you have a compound such as Na₂S, sodium sulfide, in which a potassium replaces a sodium yielding KNaS. This is called potassium sodium sulfide, potassium being named first because it is more electropositive. The compound NaHS would be sodium hydrogen sulfide for the same reason.

 

Naming Ternary/Quaternary Ionic Compounds

1. Naming Ternary Salts (Oxysalts) and their Anions (oxyanions or oxoanions)

A ternary ionic compound is a compound consisting of three or more elements in which both elements are mostly ions, a cation and an anion. Ternary ionic compounds are made up of a metal and a polyatomic ion i.e. they are formed by the combination of monoatomic cation with a polyatomic anion (oxyanion). A polyatomic anion is derived from a ternary acid. The ternary ionic compounds are those in which the cation can have multiple forms. The ternary compounds mostly containing polyatomic oxyanions whose names and formulae must be memorized. When naming these compounds, its composition must be considered.

The ternary ionic compounds are those in which the cation can have multiple forms. The ternary compounds mostly containing polyatomic oxyanions (whose names and formulae must be memorized. It is helpful to memorize some of the more important polyatomic ions to make your naming and formula writing quicker and more effective). Most of the polyatomic ions have negative charges. The exception is the ammonium ion, NH₄⁺. Occasionally there will be two polyatomic ions (e.g. NH₄NO₃, NH₄NO₂, (NH₄)₂SO₄ etc.).

1.  The cation or metal (usually the first element in the formula) is listed first and the anion (which is  generally non-metallic and polyatomic oxoanion) second.

2.    The cation takes the name of its elemental form. For example, Li⁺ would be called "Lithium". Since the cation can take on multiple charges, the charge must be written within parentheses in Roman numerals after stating the cation name. For example iron (II), iron (III) etc. 

      The older method of assigning charge or oxidation number of central metallic or non-metallic ion is to use the suffixes ous or ic for lower or higher oxidation number respectively. 

3. The anion name uses the first part of its elemental name, and

subsequently adding the suffix "-ate or ite".

For example,

NO₃⁻ would be called "nitrate". 

SO₄²⁻ would be called "sulphate". 

Examples

2. Naming Ternary Acids and their Anions (oxyanions or oxoanions)

The acids fall into three categories:

1)  Binary acids: compounds of hydrogen and a non-metal

2) Oxoacids/oxyacids:compounds of hydrogen, oxygen, and a third element

3)Organic acids: compounds that contain either carboxylic (–CO₂H) or sulphonic (-SO₃H) acid group

When a ternary compound contains hydrogen and a polyatomic anion (for example, HNO₃), its name in the pure state is hydrogen followed by the name of the anion. Pure HNO₃ has the name hydrogen nitrate. When this compound is dissolved in water, it is an acid and is named as such. HNO₃ in water solution is named nitric acid.

The rules for naming these compounds as acids follow the table. Be sure to study the table as you read the rules and notice the pattern shown in the names and formulas.

The rules for naming ternary acids are as follows:

1.  Since all ternary (or quaternary) acids have the same cation, H⁺, we don't need to name the cation and hence they have no prefix like hydro used for binary acids.

The acid name comes from the root name of the oxyanion name or the central element (non-metal) of the oxyanion plus the suffix ic or ous. Suffixes are used based on the ending of the original name of the oxyanion. If the name of the polyatomic anion ended with -ate, change it to -ic for the acid and if it ended with -ite, change it to -ous in the acid.

The name of the oxoanion of ternary acid is the root of the central element's (nom-metal) name plus the suffix ate or ite.

Thus for oxoacids, change the suffix of oxyanion, –ate to –ic or –ite to –ous.

For Example: The stem for nitrogen is nitr. Thus two oxyacids of nitrogen are named as

HNO₃, which contains the oxoanion nitrate, is called nitric acid.

HNO₂, which contains the oxoanion nitrite, is called nitrous acid.

2. The oxidation number of the non-metal in ternary acids may be high but not necessarily the highest possible. The ternary acids having higher oxidation numbers are sometimes referred to as ic-ate acids. Of the oxyacids, nitric, sulfuric, phosphoric, and chloric are "most common ternary acids." The ternary acids having two unit lower oxidation number than ic-ate acids are called ous-ite acids. Of the acids, nitrous, sulfurous, and chlorous fall into this group. Their formulas can be predicted if you have learned the formulas in the first group. The anion of an ous-ite acid contains one fewer oxygen atom than that of the ic-ate acid.

3.    As with the halogens, if there is an oxyacid in which the non-metal has an even lower oxidation number than ous-ite acid, that acid is named using the prefix-suffix hypo-ous, and its anion using hypo-ite. Of the acids, only hypochlorous is in this category. Its formula can be predicted if you know the formula of chloric acid. The anions of these acids contain two fewer oxygen atoms than the anions of the ic-ate acids.

4.   Again as with the halogens, if there is an oxyacid in which the non-metal has a higher oxidation number than ic-ate acids, that acid is named per-ic acid and its anion per-ate. Of the acids, only perchloric is important. Its formula can be predicted from the formula of chloric acid. The anion will contain one more oxygen atom than the anion of the ic-ate acid.

The prefixes per– and hypo– are used in the names of oxoacids just as they are used in the names of oxoanions.

Example

There are four oxoacids that contain chlorine.

ClO^4– –––   perchlorate ion  HClO₄ perchloric acid

ClO^3– –––  chlorate ion         HClO₃ chloric acid

ClO^2– –––  chlorite ion          HClO₂ chlorous acid

ClO^1– –––  hypochlorite ion   HClO hypochlorous acid

2. Ternary acids containing carbon/Organic Acids

Many acids contain only carbon, hydrogen, and oxygen. Acetic acid is an example. Its formula can be written as HC₂H₃O₂ or HCH₃CO₂ or CH₃COOH or CH₃CO₂H. Regardless of how it is written, there is only one acidic hydrogen in acetic acid; the other three hydrogens do not separate as hydrogen ions in aqueous solution. Notice how the acidic hydrogen is placed by itself in each of the formulas to signify this difference. Many acids, like acetic acid, contain a group of atoms bonded to a -COOH group. Only the hydrogen of the -COOH group is an acidic hydrogen. These acids are called carboxylic acids. In naming the anions of these acids, the ic of the acid is replaced by ate. For example,