The elements are represented in the abbreviated form by their symbols. Similarly, a compound is represented in the abbreviated form by its chemical formula.
The expression of the composition of a substance by chemical symbols and numerical
subscripts is called the chemical formula of the substance.
The formula of a simple substance is obtained by writing the symbol of the element and
indicating the number of atoms in a molecule of the substance by a subscript. For example, a molecule of hydrogen contains two atoms. Hence, hydrogen is represented by the formulaH2.
A molecule of ozone contains three atoms of oxygen. Hence, ozone is represented by the formula O 3.
In order to represent the chemical formula of compound, one must know what are the
elements present in the compound and what is the number of atoms of one element that combine with a definite number of atoms of the other element. For example, a molecule of water contains two hydrogen atoms and one oxygen atom. It is represented by the formula H2 O
The formula of a compound is, thus, expressed by the symbols of chemical elements
contained in the compound and by the numerical subscripts (to the lower right of each symbol).
WRITING THE FORMULA QF A BINARY AND TERNARY MOLECULAR COMPOUND
In binary molecular compounds, the atoms of the two elements are held by covalent bonds. A covalent bond, as you know, is formed by sharing of electrons between the two atoms. The number of electrons that an atom of the element contributes for sharing is known as valency of the element. Knowing the valencies of the two elements involved in the formation of a binary molecular compound, the formula of the compound can be derived.
Step 1. Write the constituent elements and. their valencies as shown below:
Valency 4 2
Element C O
Step 2. Reduce the valency numerals to simplest whole numbers by dividing by some common factor, if
Any 2 1
Step 3. Criss-cross the reduced valency numerals and write them as subscripts at bottom right hand side of the symbols.
The subscript 1 is not written. Thus, the formula of the compound is CO2. Formulae of some more compounds are given below
WRITING FORMULA OF SIMPLE IONS OR RADICALS OR POLYATOMIC IONS
Most of the inorganic compounds which are composed of metals and non-metals, contain oppositely charged parts called ions or radicals. The positively charged part is called cation or electropositive radical or basic radical whereas negatively charged part is called anion or electronegative radical or acidic radical. For example, sodium chloride is made up of Na+ ions and CI- ions. Similarly, nickel sulphate is made up of Ni2+ ions and SO 4 ions.
An ion behaves as single unit in reactions. Thus, An ion is an atom or group of atoms, carrying positive or negative charge, that behaves as a single unit in reactions.
An ion containing only one atom is known as simple ion namely simple cations, simple anion and oxoanion whereas an ion containing two or more than two atoms is known as polyatomic ion or compound ion. Thus, Na+ is a simple ion whereas SO 4 2- is a compound ion.
The charge on the ion is known as valency of the ion.
- The name of element is written.
- The oxidation state is placed after the name of the element plus the word ion. The oxidation state is put in parenthesis in capital Roman numerals, with no space in between.
- The fixed oxidation states of an element is shown. Formulae of some common electropositive ions are given in Table 12.1.
- Anions are named by adding suffix -ide for the last two or three letters in the name of the atom plus the word ion (see Table 12.2). For examlple, chloride ion, oxide ion, etc.
- The ion OH- , has the name hydroxide.
Oxoanions are anions in which the oxygen atom is bonded to a central atom. The naming is based on the oxidation number of the central atom and the number of oxygen atoms bonded to it.
- The suffix -ate replaces the last two or three letters in the name of the central atom.
- The oxidation number of the central atom is placed in parenthesis in capital Roman numerals after its name.
- The number of oxygen atoms is placed before the name of the central atom as dioxo, trioxo, tetraoxo, pentaoxo, hexaoxo, heptaoxo etc., respectively for 2, 3, 4, 5, 6 and 7 oxygen atoms. One oxygen atom is not given a prefix.
- The word ‘ion’ is added to the name.
The formula of some common electronegative ions is given in Table 12.2.
Table 12.1. Formulae and Valencies of Common Ions
Table 12.2. The Formulae of Some Common Anions
There are certain elements that exhibit more than one . valencies in their ions (or compounds). For example, iron can exist as Fe2+ or Fe3+ in its compounds. In such cases, the
name of the ion with lower valency ends with a suffix ‘ous’ while that with higher valency ends with a suffix ‘ic’.
Names and formulae of some common basic ions exhibiting variable valency are listed in Table 12.3
.Table 12.3. Some Basic Ions Exhibiting Variable Valency
WRITING THE FORMULA OF AN IONIC COMPOUND
The formula of an ionic compound represents the simplest whole number ratio of ions in it. The total positive charge on cations is equal to the total negative charge on anions. Knowing the formulae of the ions present in the compound, the formula of the compound can be written by the following steps:
Step 1. Write the formulae of the ions or radicals of the compound side by side with cation on the left hand side and anion on the right hand side.
Step 2. Enclose the compound ion (if any) in a bracket.
Step 3. Reduce the valency numerals to a simple ratio by dividing with a common factor, if any.
Step 4. Criss-cross the valencies, i.e., shift the valency numerals crosswise to the lower right-hand corner of the ions. This is done to achieve electrical neutrality.
Let us apply the above steps to write formula of calcium phosphate.
Step 1. Writing the formulae of the ions.
Ca2+ PO 3-4
Step 2. Enclose the compound ion phosphate in a bracket.
Ca2+ PO 3-4
Step 3. Not applicable, because ratio is already simple.
Step 4. Criss-cross the valencies.
Thus, the formula of calcium phosphate is: Ca3(PO4)2
The correctness of the formula can be verified by checking the charge balance as given under:
Total positive charge on 3 Ca2+ ions = 3 x (2) = + 6
Total negative charge 2 P043- ions= 2 x (- 3) =- 6
Thus, total positive charge on cations is equal to the total negative charge on anions. Hence, the formula Ca3(PO 4)2 is correct.
On the basis of above steps the formulae of some ionic compounds are given in Table 12.4.
Table 12.4. Writing Formula of an Ionic Compound
Example 12.1 Write down the formulae of
(i) sodium oxide (ii) aluminium chloride
(iii) sodium sulphide (iv) magnesium hydroxide.
Solution. (i) Sodium oxide contains Na+ and O2- ions.
The formula of sodium oxide is NazO.
(ii) Aluminium chloride
The formula of aluminium chloride is AICI3.
(iii) Sodium sulphide
The formula of sodium sulphide is NazS.
(iv) Magnesium hydroxide
The formula of magnesium hydroxide is Mg(OH)2•
Example 12.2 Write the correct formula of the following compounds
(i) sodium trioxocarbonate(IV)
(ii) Ammonium tetraoxosulphate(VI)
(iii) trioxonitrate(V) acid
(iv) Iron(III) sulphide
(v) Hydrogen sulphide.
Solution. The correct formula of the given compounds are: