Use Lewis symbols to show electron transfer between the following atoms to form cations and anions:
(a) K and S, (b) Ca and O (c) Al and N
Discuss the shape of the following molecules using VSEPR model:
BeCl2, BCl3, SiCl4, AsF5, H2S, PH3
BeCl2:
Lewis dot structure Cl: Be : Cl. The central atom (Be) has only two bond pairs and no lone pair. Hence shape is linear.
BCl3:
The central atom (B) has only three bond pairs and no lone pair. Hence shape is triangular planar.
SiCl4:
The central atom (Si) has four bond pairs and no lone pair. Hence the shape is tetrahedral.
AsF5:
The central atoms (As) has five bond pairs and no lon∈ pair. Hence, the shape is trigonal bipyramidal.
H2S:
The central atom (S) has two bond pairs and two lone pairs. Hence, the shape is Bent or V-shaped.
PH3:
The central atom (P) has three bond pairs and two lone pairs. Hence, the shape is. Bent or V-shaped.
Apart from tetrahedral geometry, another possible geometry for CH4 is square planar with four H atoms at the corners of the square and the C atom at its centre. Explain why CH4 is not square planar?
Here 1 s orbital & 3 p orbitals undergo hybridization to form sp3 hybrid orbitals .Hence, carbon atom undergoes sp3 hybridization in CH4 molecule and takes a tetrahedral shape.
For a square planar shape, the hybridization of the central atom has to be dsp2, ie, 1 s orbital, 3p orbitals & 1 d orbitals have to undergo hybridization .However, an atom of carbon does not have d-orbitals to undergo dsp2 hybridization. Hence, the structure of CH4 cannot be square planar.
Moreover, with a bond angle of 90° in square planar, the stability of CH4 will be very less because of the repulsion existing between the bond pairs. Hence, VSEPR theory also supports a tetrahedral structure for CH4.
In a molecule as we increase the number of electrons shared between two atoms, there will be an increase in bond order, also there will be an increase in the strength of the bond, and decrease the distance between nuclei. For example,
Bond |
No. of electron |
Bond order |
Bond strength |
Bond Length |
Single |
2 |
1 |
Weakest |
Longest |
Double |
4 |
2 |
- |
- |
Triple |
6 |
3 |
Strongest |
Shortest |
Although geometries of NH3 and H2O molecules are distorted tetrahedral, the bond angle in water is less than that of ammonia. Discuss.