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Chemical Bonds.

posted Jan 28, 2015, 7:21 PM by Ranmini Perera   [ updated May 2, 2016, 5:35 PM by Upali Salpadoru ]

Why do atoms bond? Why can’t they stay single?

There are some atoms that remain single. They are the members of the Noble family.

Sometimes called, “The inert gases”.

Nobles.jpg

Fig.1 A family of Nobles.

What make these atoms so stable? It can easily be observed that the outermost shell in these possess  8 except in He. 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”.

There are two main types of bonding. One method is to share the valency electrons and the other is to form ions and get attracted by the electrical charges. The two kinds of bonds are the Co-valent bond and the Ionic bond.

 Take the 

Cl 2.jpg
case of Chlorine gas. Cl-17.; 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.2  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.CH4.jpg

Fig.3 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.

 






Fig.4. Transfering a Lewis 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.







More on Co-valent Compounds






Fig. 5 Carbon atom takes 4 chlorine atoms.


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 6 A pair of sharing electrons is shown by a line.


If you observe the diagram  carefully you can see that there are 8 electrons in the outer shell 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. This kind of a bond is called a co-valent bond.


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 7 Hydrogen gets the structure of Helium for stability.

   Double Bonds.

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




Fig.8  There are two double bonds in this molecule. 


A comparison of covalent compounds with the Ionic compounds.

  Exceptions to rule

There are a few exceptions to the Octet rule. Here is one. H3PO4.jpg

There are 5 lines around P. Counting each line to a pair there are 10 electrons. This is a case of expansion of the Octet.



 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.  Methane molecule. CH4





Ionic Compounds

]
Fig .9  Master Hydrogen and Miss chlorine.

If Miss Chlorine has only one free hand and Master Hydrogen also has only one free hand, there is no other way for them to dance. One hand held by another; only a single bond. 

The extra electron chlorine has and the extra proton Hydrogen has is compared to hands. 

When they combine the electrical charges get cancelled. It can be shown as below.

                                                 Hydrogen  +  Chlorine   =   Hydrogen chloride

Positive charge of H cancels with the negative charge of Cl.

Miss Oxygen has both her hands free.  She can dance with two Hydrogens.
Fig.10  Miss Oxygen takes two Hydrogen atoms.

Aluminium atom may be compared to a silver coloured monkey. The three extra protons it will have after donating 3 electrons can be compared to 3 hold fasts; they being the two hands and the tail. So according to the illustrations two silver monkeys can dance with three girls.             

Fig.11  Three girls holding hands and tails.

In order to get the formula of a compound it is not necessary to consider, boys and girls.All metals ,Hydrogen and Ammonium group have a certain number of positive charges. Acid groups, such as chlorides, nitrates and sulphates have negative charges. The diagram here explains how these charges cancel out.


ids have a red back ground while the Alkalies are blue.




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. Do not conduct electricity.

Mostly solids generally crystalline.

There are strong forces between molecules.

Soluble in water.

Solutions conduct electricity.



1.0   Give the formula of these compounds . Hydrogen raw has been completed.


Name

oxide

hydroxide

chloride

nitrate

sulphate

carbonate

sulphide

Ions

O--

OH-

Cl-

NO3-

SO4--

CO3--

S--

NH4+

 

 1

H+

 H2O

 HOH

 HCl

HNO3 

H2SO4 

 H2CO3

 H2S

K+

 7    

10 

11 

12 

13 

Na+

14 

15 

16 

17 

18 

19 

20 

Ca++

21 

22 

23 

24 

25 

26 

27 

Mg++

28 

29 

30 

31 

32 

33 

34 

Al+++

35 

36 

37 

38 

39 

40 

41 

Zn++

42 

43 

44 

45 

46 

47 

48 

Fe++

49 

50 

51 

52 

53 

54 

55 

Fe+++

 56

57 

 58

59 

 60

61 

62 

Pb++

63 

64 

65 

66 

67 

68 

69 

Cu++

70 

71 

72 

73 

74 

75 

76 

Ag+

 77

78 

79 

80 

81 

82 

83 



2. a.   Give the simple co-valent bond diagrams for the following.
              i.  Hydrogen and Iodine,    ii.  Phosphorus and fluorine.    iii.  Nitrogen and hydrogen.       
2. b.   Give the dot and cross Lewis diagrams for the above.


ANSWERS
1.0       NH4OH,  2. NH4Cl,  3. NH4NO3,  4. (NH4)2SO4,  5. (NH4)2CO3,  6. (NH4)2S,  7. K2O,  8. KOH,  9. KCl,  10.  KNO3,   11.K2SO4,   12. K2CO3,  13.  K2S,  14.  Na2O,  15. NaOH,   16. NH4Cl,  17. NaNO3,  18.Na2SO4,   19. Na2CO3. 
If you are still not sure of the rest please go to Ionic compounds .

2.0    
    

Q.1.0

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

       i.  C and F.     ii.   N and H,   iii. H and S,   iv.  C,H and Cl,   v.  P and H,  vi. C and O,   
 b.  Give the structural formala of the above molecules.
                        a. 3x6-18   and b. 3x6=18 .                  36 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-

                                                                                        5x5=25 Marks.

Q. 3.0

  Show how these metallic ions combine with the Hydroxyl ion (OH_) to form molecules.

     i.  Sodium,    ii.  Magnesium,  iii,  Aluminium  iv. Lead   V. Silver.

                                                                        5x5 = 25. Marks.

Q. 4.0

  Draw two different forms of structural formulae for this molecule.  C4H10.

                                                                               2x7=14 marks.

For the answers click  Answer page.Answer page-

 
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