Chemical Formula - Definition, Types & Examples

Chemical formula is the way of representation of the molecule which gives information about all the elements present and their proportion in that molecule.

Key words: Chemical formula,  Molecular formula, Empirical formula, Condensed formula, Displayed formula, Structural formula, Skeletal formula, Hill Notation.

Introduction

It is a simplest way to represent molecule. In organic chemistry chemical formula of the molecule is written by using element symbols, numbers, dashes, parentheses, brackets and plus / minus signs. A chemical formula does not consist of words and it is not actual name of the molecule.

There are various types of chemical formulae available in organic chemistry and according to the situation these formulae are used to indicate identity of the molecule. The types of chemical formulae are;

1.  Molecular formula  
2.  Empirical formula
3. Condensed formula
4.  Displayed formula
5. Structural formula
6. Skeletal formula

Molecular formula

A molecular formula of the molecule indicates total number of each atom present in the molecule.

For example, molecular formula of butane is C₄H₁₀. This suggests that butane has 4 carbon atoms and 10 hydrogen atoms.

Take another example, molecular formula of glucose is C₆H₁₂O₆. That means glucose molecule has 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms.

Empirical formula

An empirical formula of the molecule indicates the ratio of each element present in the molecule.

For example, butane has molecular formula C₄H₁₀. It has the empirical formula C₂H₅ because Carbon: Hydrogen ratio is 2:5.

Let’s consider another example, glucose has molecular formula C₆H₁₂O₆ and it has empirical formula CH₂O because Carbon: Hydrogen: Oxygen ratio is 1:2:1.

Condensed formula

A condensed formula is a text based formula where all the carbon atoms written in a line along with other atoms which are attached to respective carbon atoms.

For example butane has condensed formula as CH₃CH₂CH₂CH₃. This formula indicates that butane has TWO CH₃ and TWO CH₂ units in the structure of molecule.

Consider another example; acetic acid has condensed formula as CH₃COOH. This formula indicates that acetic acid has CH₃ and COOH units in the structure of molecule

Displayed formula

A displayed formula is a graphical representation of structure of the molecule. It shows all the atoms and bonds present in the molecule. In this formula of molecule each line represents a single bond between two atoms.

For example, butane has displayed formula as                      

   

Acetic acid has displayed formula as            

Structural formula

It is similar to displayed formula. The main difference is that here carbon-hydrogen bonds are often omitted. 

For example, butane has structural formula as 

Acetic acid has structural formula as

Skeletal formula

In this type of formula all carbon atoms and most of the hydrogen atoms are removed. The hydrogen which are attached to atoms other than carbon atoms are shown in the structure. The functional groups and the atoms other than carbon and hydrogen are shown in the structure. The carbon chain is shown as zigzag line.

For example, butane has structural formula as 

Acetic acid has structural formula as 

General formula

A general formula is often used for compounds which are differing from each other by a constant unit. It is useful to show homologous series of compounds.

For example alkanes has general formula of C_nH_2n+2, (where n ≥ 1). It gives homologous series of alkanes like methane, ethane, and propane for n = 1-3.

Similarly alcohol has general formula of C_nH_(2n+1)OH (Where n ≥ 1). It gives homologous series of alcohols like methyl alcohol, ethyl alcohol, and propyl alcohol for n = 1-3.

Organic compound	General formula
Alkane	CnH2n+2 (n ≥ 1)
Alkene	CnH2n (n ≥ 2)
Alkyne	CnH2n-2  (n ≥ 2)
Alcohol	CnH(2n+1)OH (n ≥ 1)
Amine	CnH(2n+1)NH2 (n ≥ 1)

Table 1

Hill Notation

Hill notation is the systematic way of writing condensed formula, molecular formula and empirical formula of the compound. It was introduced by Edwin A. Hill in 1900. According to this system the carbon atom should be written first then hydrogen atom followed by all the other atoms in alphabetical order. When the carbon atom is not present in the molecule then all the atoms including hydrogen atom are written in alphabetical order.

For example, empirical formula of methyl iodide is written as CH₃I, here carbon is written first than hydrogen followed by iodine.

Empirical formula of acetone is written as C₃H₆O here also carbon is written first than hydrogen followed by oxygen.

Some other formulae which are written according to Hill notation are given in Table 2.

Compound	Formula
Hydrochloric acid	HCl
Acetic acid	C2H4O2
Glucose	C6H12O6
Formaldehyde	CH2O
Dichloromethane	CH2Cl2
Tetrahydrofuran	C4H8O
Dimethyl sulfoxide	C2H6OS

Table 2

To summarize this topic, a Chemical Formula is the simple way to describe a chemical compound. There are various kinds of the chemical formulae are used in organic chemistry to represent the compounds.

For example

Name	Hexane
Molecular formula	C6H14
Empirical formula	C3H7
Condensed formula	CH3CH2CH2CH2CH2CH3
Displayed formula
Structural formula	CH3-CH2-CH2-CH2-CH2-CH3
Skeletal formula

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