1. Explain the geometery shape of molecule Pcl5 and Pcl3 step wise?
  2. write the basis of VESPR theory and explain its significant using suitable eg.
  3. Write the geometry of Sicl4 , Bacl3.
  4. Explain the term overlapping and write down positive and zero overlap using  suitable eg.

 

I am answering three questions.

 

1. a) In the molecule PCl5, The electronic configuration of P atom is 1s2, 2s2, 2p6, 3s2, 3p3. The valency of P is 5. In PCl5, P has 5 single bonds and no lone pairs.

This suggests us to use sp3d hybrid orbitals on phosphorus. Each chlorine atom (valence shell configuration 3s23p5) has one singly occupied 3p orbital. The P–Cl bonds are formed by the overlap of a phosphorus sp3d hybrid orbital with singly occupied chlorine 3p orbital. Each Cl atom holds three lone pairs.

 

Lewis dot structure of PCl5

Trigonal bipyramidal structure of PCl5

 

Hence in PCl5, the number of electron pairs is 5.The bond pairs is 5. It does not have any lone pair of electrons. It has sp3d hybridyzation. It has trigonal bipyramidal shape.

 

b) In the molecule PCl3, The electronic configuration of P atom is 1s2, 2s2, 2p6, 3s2, 3p3. The valency of P is 5. In PCl3, 

P has 3 single bonds and 1 lone pair (pair of unshared electrons).

This suggests us to use sp3 hybrid orbitals on phosphorus. Each chlorine atom (valence shell configuration 3s23p5) has one singly occupied 3p orbital. The P–Cl bonds are formed by the overlap of a phosphorus sp3 hybrid orbital with singly occupied chlorine 3p orbital. Each Cl atom holds three lone pairs.

 

Lewis dot structure of PCl3

 

Trigonal pyramidal structure of PCl3

 

Hence in PCl3, the number of electron pairs is 3.The bond pairs is 3. It has 1 lone pair of electrons. It has sp3 hybridyzation. It has trigonal pyramidal shape.

 

 

 

2. VSEPR means Valence Shell Electron Pair Repulsion. This vsepr theory explains how molecules and ions behave. It is used to determine the shapes of molecules and ions.

 

1. The principle of VSEPR is that the valence electron pairs surrounding an atom mutually repel each other, and will therefore adopt an arrangement that minimizes this repulsion, thus determining the molecular geometry.

2. There are two types of valence shell electron pairs

 (i) bond pairs – shared by two atoms, occupies less space and cause less repulsion.

 (ii) lone pairs – not involved in bonding and are in attraction with one nucleus, occupies more space and cause more repulsion.

These two valence shell electron pairs affect the geometry of the molecule.

3. In vsepr, electron pairs in multiple bonds are considered as single super pair.

4. The shape of the molecule is determined by number and type of valence shell electron pairs around central atom.

5. The VSEPR rules mainly predict the layout of electron pairs surrounding one or more central atoms in a molecule, which are bonded to two or more other atoms. The geometry of these central atoms in turn determines the geometry of the whole molecule.

 

Please refer to the table where molecule is represented by AXE notation for better understanding (as it is very big and I cannot give that here).

 

Please find the method by which VSEPR theory is used in determining the geometry of a molecule.

1. Determine the central atom (atom capable of making most bonds).

2. Draw the electron dot structure and bar diagram.

3. Determine the molecular geometry using all electron pairs and atoms around the central atom.

4. Modify the geometry to determine the molecular shape if non-bonding electron pairs exist by ignoring them.

 

Let us take example of BeH2

1. Central atom Be – only one atom.

2. Electron dot structure  H : Be : H  (Be violates the octet rule this is exception)

  Bar diagram H—Be—H

3. The two atoms attached to central atom could be spread furthest apart by 180 degrees.

4. Ignoring any unbonded pairs (not necessary here). The shape is linear.

 

 

3. Geometry of SiCl4

 

In SiCl4, the Si central atom has electronic configuration 3s2, 3p2 and has 4 valence electrons. In SiCl4, Si has 4 single bonds and no lone pairs.

This suggests us to use sp3 hybrid orbitals on silicon with tetrahedron geometry. Each chlorine atom (valence shell configuration 3s23p5) has one singly occupied 3p orbital. The Si–Cl bonds are formed by the overlap of a silicon sp3 hybrid orbital with singly occupied chlorine 3p orbital. Each Cl atom holds three lone pairs.

 

Lewis dot structure of SiCl4

 

Tetrahedron structure of SiCl4

 

Hence in SiCl4, the number of electron pairs is 4.The bond pairs is 4. It does not have any lone pair of electrons. It has sp3 hybridization. It has tetrahedron geometry.

 

Geometry of BaCl3

This compound is not likely to occur. Ba is in group 2A and will lose 2 electrons and become Ba 2+ Cl is in group 7 and can only accept one electron, so BaCl2 would be correct but BaCl3 is impossible.

In each case the number of electrons gained and the number of electrons lost must be equal or the charges must balance out.

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