Chemistry‎ > ‎

Chemical Bonds.

posted Jan 28, 2015, 7:21 PM by Ranmini Perera   [ updated Sep 11, 2019, 2:27 AM by Upali Salpadoru ]

Why do atoms bond?

Why can’t they stay single?

Fig.1. Miss Oxygen dancing with two Hydrogens.


Fig.2. First three periods (rows) of the Mendeleve's Periodic table.

Here the atoms of all elements are arranged according to their Atomic masses.

Atoms are placed in columns according to the number of electrons in the outer shell

All atoms, where the outer shell is complete is in the "O" column.

These are called "Noble gases". Do you know why?

( According to another arrangement of this table Noble gases are in column 18.)

Noble elements always remain single.

They are sometimes called "Inert Gases"

The electron structure of 5 of them are given below.


Fig.3 The electron structure of Nobles.

What make these atoms so stable?


It can be observed that the outermost shell in these possess  8 except in Helium.

Helium has 2, which is the maximum it could have.

 So it has been suggested, that generally all atoms try to achieve a similar structure by bonding.
This is known as the “Octet rule”. (Octet means eight.)


This is a term useful to know with regard to bonding. It generally means the number of Hydrogen atoms a particular atom can displace or combine with.
It can also mean the number of electrons an atom can donate or gain in inorder to complete the octet..

Fig.4. A Carbon atom can take 4 Hydrogen atoms.
This shows that the the valency of carbon is 4.

Types of bonding.

There are two main ways to achieve the octet.

The two kinds of bonds are the Co-valent bond and the Ionic bond. What are they?

 Co-valent bond:- Ionic bond:-
 Octet is achieved by sharing electrons. Octet is achieved by donating or gaining electrons.

In both cases only the valency electrons participate.

Co-valent bonding.

Take the case of Chlorine gas. Atomic mass Mass of Cl-17.;

Cl 2.jpg
The electron configuration is 2, 8 and 7.

When two chlorine atoms bond together they obey the octet rule. This sort of a bond formed by sharing electrons is known as a covalent bond.

Fig.5  Sharing a pair of electrons by 2 chlorine atoms.

A very good example is how Carbon combines with Hydrogen.  C- 6. E-configuration= 2, and4.  H -1.


Fig.6 How carbon bonds with Hydrogen.

One Carbon atom takes 4 Hydrogen atoms to complete the octet.

(This type of dot diagrams are known as Lewis diagrams as they were introduced by Gilbert Lewis in 1916.


Transferring a dot diagram to a line diagram.

The sharing of a pair of electrons can be shown by a short line. According to this method methane can be shown like this.

Fig.7. Transfering a Lewis diagram to a line diagram.

More examples on Co-valent Compounds.

  Let us now take the case of Carbon and chlorine. In this diagram the electrons of chlorine are shown in green while that of Carbon in blue

   Fig 9 A pair of sharing electrons is shown by a line.Fig. 8 Carbon atom takes 4 chlorine atoms.

 If you observe the diagram  carefully you can see that there are 8 electrons in the outer shell of Carbon, along with that of chlorine. Similarly Chlorine, which has only 7 e-  in the last shell has completed the octet with the e- of Carbon. 

 Carbon is not the only element that form co-valent bonds. The example given above is a bit complex. We can give a very simple example with Hydrogen. Hydrogen atom has only one electron at the lowest energy level. At this level there has to be two e- to gain stability. The figure below shows how two Hydrogen atoms achieve the inert gas structure of Helium by sharing electrons. Colours are used only to show the difference.

Fig 10 Hydrogen gets the structure of Helium for stability.

Valency to be compared to links.

Fig. 11. Oxygen has two links, while Hydrogen has only one.

Double Bonds.

. A double bond consists of two pairs of sharing electrons.
Carbon and Oxygen form such a bond. 

Fig.12  There are two double bonds in this Carbon dioxide  molecule. 

 Molecular Models
    The above diagrams of structural formulae may give the impression that these molecules are flat. But they are actually 3 dimentional. If you make models with coloured plasticene and match sticks you can get something closer to real shape.  Here is an example of the Methane molecule.

Fig. 13. Methane molecule. CH4

Ionic Compounds

     When an Atoms donates a valency electron it gets a positive charge due to the unbalanced proton.
   Example:-          H - e-  ----->  H+.
      Hydrogen atom - electron  -----> Hydrogen ion.
      Hydrogen ion has a positive charge of 1+  
      It is an  "an-ion" as it gets attracted to the anode during electrolysis.

      Ca - 2 e-    ---->  Ca 2+
       Calcium ion has a charge of minus 2.

     When an atom accepts an electron it gets a negative charge. 
   Example:-        Cl + e- ----->  Cl - 
     Chlorine + electron  ------>  Chloride ion.
     Chloride ion has a charge of minus one.
     It is a "cat-ion" as it is attracted to the anode during electrolysis.

    Un equally, electrically charged ions get attracted to each other forming ionic bonds.
    Example:-       H+    +    Cl -   =    HCl.

                           Hydrogen  +  Chlorine   =   Hydrogen chloride         
                              Ca 2+    +    Cl - +    Cl -   ----->  CaCl2
                 Calcium ion with two chloride ions forms the  Calcium chloride molecule.

                             The number of electrons gained or lost during the formation of an ionic bond.
   Using the Periodic table given in Fig.2,  we can predict the charges a particular atom can get.

 All atoms coloured red or some colour close to red can donate electrons.
 The blue ones accept electrons.
  Darker the colour, greater the reactivity.
  Groups give the valency.
  In case of red ones, metals,  the valency is equal to the group number.
  In the cassee of blue ones the valency is equal to 8 - group no.
Obtaining Molecular formulas.
    So far we have dealt with ions formed from single atoms. But some compounds break up forming complex ions; consisting of more than one element.
      When CO2 , SO2 etc,dissolve in water these are the ions produced. 
      H2O    +  CO2    ------->    H2CO3   --------->   H+ H+  +  CO3 2-
      Carbonic acid,   H2CO3   is made up of he three ions:-   Namely 2 hydrogen ions and one carbonate ion. 
      Sulphuric acid contains  two  H+  and a suphate ion. 
            H2SO4     --------->  2 H +  SO4 2-
         These ions of a Salt, Acid or an aAkali are also known as  "radicals".
  In this chart Cat-ions or the positive ions or radicals are in red.
  The negative ions or radicals are in blue.
   It is up to you to get the formulas of the compounds that may form.
   Some formulas are given as examples.




















NH4Cl,NH4NO3, (NH4)2SO4, (NH4)2CO3(NH4)2S,

















Ca(OH)2Ca (NO3)2



















   Highlighting can make some answers visible.

A short cut to get the formula of a compound.

   The digits cross over or cancel.

 [Ca++]  + [OH-]+ [OH-] ----> Ca(OH)2  
In order to cancel 2 charges of the Ca++ ion there should be 2 negative ions.                                  

A comparison of covalent compounds with the Ionic compounds.

Co-valent compounds

Ionic compounds

1. Simple compounds are generally gases or liquids.

(This is due to low forces between molecules)

2. Generally not soluble in water.

3. Solutions do not conduct electricity.

Mostly solids generally crystalline.

There are strong forces between molecules.

Soluble in water.

Solutions conduct electricity.


             Multiple Choice Questions.

 1.0    Why Ozone O3 becomes a compound defying the general definition of a  compound?

    A- Oxygen atoms are not held together by chemical bonds.

    B- Ozone contains atoms of one element.

    C-  It does not form ions.

    D-  It is unstable and easily breaks down .

2.0   Why does Helium remain single while Hydrogen form many chemical bonds.

     A-  Hydrogen does not have neutrons.      B- Helium is a noble gas.  
     C-  Helium has the maximum number of electrons in the outer shell.
     D-  Helium has a mass `number of 4 while that of hydrogen is one.

3.0    Which property of an atom is closely related to the 'Valency'?
      A-  Number of protons and electrons .         B - Number of Protons .  
      C-  Total Electron number           D- Number of electrons in the outermost shell.

4.0  The electron arrangement of P (Phosphorus ) is 2-8-5. Give the formula of the compound it can make with Hydrogen.

  A- HP3     B- PH3    C-  P3H5    D- P5H3

    For  questions 5 to 8.

 5.0  Which gives the Lewis structure of methane?
 6.0  Which gives the structural formula of ethane?
 7.0  Which may not be a correct compound of carbon?
 8.0  Which 2 formulas indicate the same compound?
              A-  A & B.        B-   A & C        C-  A and D,        D-  B & C.

 9.0.   A pure compound X has the following properties. Which could be a correct statement regarding this? 1.  An aqueous solution of it conducts electricity.
                    2.  It has a very high melting point.
     3.  Crystals form when the solvent evaporates.
   A.  It is an ionic compound.   B- It is a co valent compound.
   C.   It is a metal.                  D- It is an organic compound.
10.0   Atomic numbers of two elements X and Y   are X= 13, and Y = 16.
         A - They make a compound  X3 Y 2.
         B- They make a compound soluble in water.
         C-  They make a co valent compound.
         D -  The compound they make should be a gas.
                                                                                        4 X 10  = 40 marks.

                                      Click nere for   Answers

         General Questions.


 a.  Give the Molecular formula of the molecules that may form with these atoms:  

       i.  C and F.        ii. H and S,   iii.  Carbon, Hydrogen and chlorine.,   iv.  P and H,    v. C and O,   
 b.  Give the structural formula of the above molecules.
                                                                                         2 X 10 =20 marks.
Q. 2.0

   Show how these ions combine with the Hydrogen ion  to form molecules using Lewis diagrams or line diagrams.

     i.    O 2-,   ii.   Cl-,   iii.  (SO4) 2-  iv. (CO3) 2-,  v. (PO4) 3-

                                                                                            2 X5 =10 marks,

 Q. 3.

  Show how these metallic ions combine with negative ions shown to form molecules.

 Cl- CO32_ PO43_NO3-
  Na + NaCl Na2CO3 Na3PO4 NaNo3
 Mg2+ MgCl2 MgCo3 Mg3(PO4)2 Mg(No3)2
 Pb2+ PbCl2 PbCo3 Pb3(PO4)2 Pb(No3)2
 Al3+ AlCl3 Al2 (CO3)2 Al2(PO4)3  
  Ag+ AgCl Ag2CO3 Ag3(PO4) 
                                                                                20 marks.

Q. 4.0

  Draw two different forms of structural formulas for this molecule.  C4H10.                                                                                                                      10 marks.