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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


Formation of cations and anions by the Lewis given as,

(a) K and S


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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.



H
2S:

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.




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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?


Electronic configuration of carbon in the excited state is

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.

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How do you express the bond length is terms of bond order?

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

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Although geometries of NH3 and H2O molecules are distorted tetrahedral, the bond angle in water is less than that of ammonia. Discuss.


In NH3, there are three bond pairs and one lone pair on N-atom. There is only one lone pair on N-atom to repel the bond pairs. In H2O, there are two bond pairs and two lone pairs on O-atom. There are two lone pairs on O-atom to repel the bond pairs. Hence, the repulsions on bond pairs are greater in H2O than in NH3 and hence the bond angle is less.

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