Formula of Molecular And Ionic Compound

1.0Framing of Formulae of Compounds With The Help of Valency

A compound is a substance formed by the combination of atoms of two or more elements. To be able to frame formulae, one should be thorough with the valencies.

The valency of Mg = 2 and that of N = 3 and the formula is Mg₃N₂. So, the number of magnesium atoms is equal to the valency of Nitrogen (3), and the number of nitrogen atoms is equal to the valency of Mg (2). So, to frame this formula, write Mg and N and write their respective valencies just below them as:

Mg N

2 3

Then place 3 near Mg and 2 near N. In other words, interchange their valency numbers.

Naming of Ionic Compounds

Cation is always named 1^st followed by the anion. The number of cations and anions are not written in the name. For example, Al₂(SO₄)₃ is called aluminium sulphate and not dialuminium trisulphate.

2.0Some Ionic Compounds

While writing the formula of an ionic compound the metal is written on the left-hand side while the non-metal is written on the right-hand side. The name of the metal remains as such but that of the non-metal is changed to have the ending 'ide'.

For example, MgO is named magnesium oxide, KCl is named potassium chloride etc.

3.0Naming of Molecular Compounds

The ones formed by the combination between two different non-metals, are written in such a way that the less electronegative element is written on the left-hand side while the more electronegative element is written on the right-hand side. Fluorine(F), Oxygen(O), Nitrogen(N), Chlorine(Cl), Sulphur(S) are more electronegative elements. In naming molecular compounds, the name of the less electronegative non-metal is written as such but the name of the more electronegative element is changed to have the ending 'ide'.

For example, H₂S is named as hydrogen sulphide.

When there is more than one atom of an element present in the formula of the compound, then the number of atoms are indicated by the use of appropriate prefixes (mono for 1, di for 2, tri for 3, tetra for 4 atoms etc.) in the name of the compound.

For example, CO₂ is named as carbon dioxide, CCl₄ is named as carbon tetra chloride. 

The prefixes are also needed in naming those binary compounds in which the two non-metals form more than one compound (by having different numbers of atoms).

For example, two non-metal, nitrogen and oxygen, combine to form different compounds like nitrogen monoxide (NO), nitrogen di-oxide (NO₂), di-nitrogen tri-oxide (N₂O₃) etc.    

But, if two non-metals form only one compound, then prefixes are not used in naming such compounds. For example, Hydrogen and sulphur combine to form only one compound H₂S, So, H₂S is named as hydrogen sulphide and not hydrogen monosulphide.

4.0Writing the Formula of Molecular Compound

Steps

The steps to be followed for writing the formula of molecular compound are 

1. First, write the symbols of the elements contributing to the compound. As a convention, the less electronegative element is written on the left-hand side while the more electronegative element is written on the right-hand side.

2. Then, below each symbol, write its corresponding valency.

3. Finally, we exchange the valencies of the combining atoms that is, with first atom, we write the valency of the second atom and with second atom, we write the valency of the first atom, the valencies are to be written as subscripts to the symbols. 

4. If the valencies have any common factor, then the formula is divided by the common factor. This gives the required formula of the compound.

For example, to work out the formula of hydrogen sulphide. 

(1) Hydrogen sulphide compound is made up of hydrogen and sulphur elements. So, first we write down the symbol of hydrogen and sulphur.

(2) The valency of hydrogen is 1 and the valency of sulphur is 2. So, below the symbol H, we write 1 and below the symbol S, we write 2.

Symbol H S

Valencies 1 2

Cross-over valencies

5. We now cross-over the valencies of H and S atoms. With the H atom, we write the valency of S (which is 2) so that it becomes H₂. With the S atom, we write the valency of H (which is 1) so that it becomes S₁.  Now, joining together H₂ and S₁ the formula of hydrogen sulphide becomes H₂S₁ or H₂S (This is because we don't write the subscript 1 with an atom in a formula).

5.0Points to Remember For Writing Chemical Formula

When the subscript is 1, it is ignored.

The compound radical is written in parenthesis when the subscript is 2 or greater.

Whenever possible, subscripts are simplified by dividing by the highest common factor (HCF).

6.0Writing the Formula of Ionic Compound

Steps

1. First, write the symbols of the ions from which the ionic compound is made. As a convention, the cation is written on the left side while the anion is written on the right side. 

2. Then, the valencies of the respective cation and anion are written below their symbols. 

3. The valencies of the cation and anion are exchanged. The number of cations and anions in the formula of the compound are adjusted in such a way that total positive charge of cation becomes equal to the total negative charge of the anion making the ionic compound electrically neutral. 

4. The final formula of the ionic compound is then written but the charges present on the cation and the anion are not shown.

Note : In the formula of the compound, the valency '1' is normally omitted. For example, NH₃ and not as N₁H₃. In case there is some common factor in the formula, it has to be taken out. For example, CO₂ in place of C₂O₄ is used.

For example,

(a) Molecular compounds

(b) Ionic compounds

Significance of Molecular Formula of A Substance

(i) It tells the name of the substance.

(ii) It tells about the names of the different elements present in the substance.

(iii) It represents one molecule of the substance.

(iv) It tells about the number of atoms of each element present in 1 molecule of substance.

(v) It tells about the atomicity of the substance.

7.0Chemical Formula of Some Compounds

8.0 Also Read