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Wednesday, 21 October 2015

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 4

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 4



Q . What are ionic compounds? Explain their characteristics:
Ans: Ionic Compound:
The compounds whose atoms are bonded together by ionic bonds, are known as ionic compounds. For example. NaCl, CaO. Na2CO3 etc.
Characteristics of Ionic Compounds:
Following are some important characteristics of ionic compounds.
  1. As ionic compounds are formed by charged ions, therefore they easily dissolve in polar solvents.
  2. In solid state, they are non—conductors of electricity but in matter state or in solution form, they are very good conductors of electricity.
  3. Such compounds are non-directional. As they are non—directional therefore, they do not show isomerism.
  4. The reaction of ionic compounds are very fast.
  5. They are mostly found in solid state.

Q : What are covalent compounds? Explain their characteristics?
Ans: Covalent Compound: The compound whose atoms are bonded together by covalent bond, are known as covalent compounds. Eg. HCL, H2O, H2SO4 etc.
Characteristics of Covalent Compounds:
Following are some important characteristics of covalent compounds.
  1. Most of the covalent compounds are insoluble in polar solvents.
  2. They are mostly found in liquid state.
  3. They are non- conductors of electricity.
  4. They are directional. As they are directional, therefore they show isomerism.
  5. Their reactions are slow.

Valence Shell Electron Pair Repulsion Theory

VSEPR- Theory

To explain the geometry of covalent molecules, R.J. Gillespie & Nyholm, put forward VSEPR theory. This theory explains that every molecule has a central atom. The central atom plays a key role in the geometry of molecule. The electron pairs present in the valence shell of the central atom are of two types i.e. bond pairs and lone pairs. These electron pairs repel each other to have maximum separation from each other. At an eagle of 180 Degree , the electron pair, in the valence shell of the central atom, are the most stable. The geometrical shapes of the molecules actually result from the tendency of electron pairs to remain at a maximum distance apart.

Postulates:

The main postulates of VSEPR theory are:
  1. Every molecule has a central atom, which plays a key rule in the molecule.
  2. There are two types of electrons pairs in the valence shell of the central atom. i.e i) Lone pair ( l.p ) and ii) Bond Pair ( b.p )
  3. The electron pairs presents in the valence shell of central atom, repel each other ie tend to be at maximum separation from each other and thus cause for the various geometries of molecules.
  4. As lone pairs are under the influence of only one nucleus (ie nucleus of central atom) therefore they occupy more space. On the other hand bond pair of electron, are under the influence of two nuclei, therefore they occupy less space.
  5. The order of repulsion of the electron pairs in the valence shell of central atom is, L.P.----L.P.> L.P.—B.P.> b.P.—b.p.
  6. The repulsion between the electron pairs in the valence shell of central may be called as vander Waal"s Repulsion or Exchange Repulsion:
  7. The effect of a bond pair ( bonding electron pair ) decreases with increasing electronegativity of an atom forming a molecule. In case of a double covalent bond, the two electron pairs or in case of triple covalent bond , the three electron pairs, take up more space than the electron pair of a single covalent Bond. However, the two electron pairs of a double bond or three electron pairs of a triple bond, act as a single electron pair of a single bond.

Applications:

As covalent bond is directional in nature, thus every covalent molecule will have a particular geometrical shape which can be best explained by VSEPR theory. As the valence shell of an atom can have a maximum of four electron pairs for stability point of view, then according to VSEPR theory, depending upon the nature and no. of electron pairs in the valence shell of central atom of a molecule, various geometries are possible.

According to VSEPR theory, the geometrics of different molecules can be explained as:

1. Beryllium Chloride ( BeCl2 ):
In BeCl2 molecule be is the central atom. There are two electron pairs in the valence shell of Be in BeCl2. Both of these electron pairs are bond pair, therefore according to VSEPR concept, BeCl2 has a linear Geometry.

2. Boron Trifloride ( BF3 ):
In BF3 molecule, "B" ( boron ) is the called atom and it has 3 electron pairs in its valence shell. All these three electron pairs present in the valence shell of the central atom ( B ) of BF3, are bond pairs. Therefore BF3 molecule has a triangular planner geometry.

3. Sulphur Dioxide ( SO2 ):
In SO2 , sculpture (S) is the central atom and it has three electron pairs in its valence shell. Out of these three two are bond pairs while the third one is lone pair. In SO2 there is one single covalent Bond and one double covalent bond. But the tow electron pairs of the double bond are considered as a single electron pair b/c they behave as a single electron pair according VSEPR concept. Due to the presence of three electron pairs in the central atom of SO2 the bond angle should be 120 degree but due to the presence of a lone pair, the behaviour of all the electron pairs is not same and thus due to the presence of this lone pair, it offer more repulsion, therefore the bond angle of SO2 is less than 120 degree and it get an angular geometry.

4. Methane ( CH4 ):
Carbon ( C ) us the central atom is CH4 molecule. Therefore are four electron pairs in the valence shell of C in CH4. All these four electron pairs are bond pairs. Therefore according to VSEPR concepts, CH4 has a tetrahedral geometry with a bond angle of 109 degree.

5. Ammonia ( NH3 ):
Nitrogen ( N) is the central atom in NH3 molecule and it has 4 electron pairs in its valence shell. Three of them are bond pairs and only one is a lone pair. As lone pair—bond pair repulsion is greater than bond pair—bond pair repulsion. Therefore NH3 gets pyramidal geometry instead of tetrahedral and the bond angle is 107.5 degree instead of 109.5 degree. So we can say that NH3 molecule is slightly distorted tetrahedron, due to the presence of a lone pair of electrons

6. Water ( H2O ):
Oxygen ( O) is the central atom in H2O molecule and has 4 electron pairs in its valence shell. Two of them are lone pair and 2 are bond pairs. As according to VSEPR concept, lone pair— lone pair repulsion is maximum, therefore H2O has got angular geometry with a bond angle of 104.5 degree

Drawbacks or Short Comings of VSEPR Theory:

Although VSEPR theory gives a successful explanation of the shape and geometry of molecules but it totally fails in the following areas. This theory gives no idea about the formation of bond.

Written by: Asad Hussain

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