68.CHEMICAL BONDING (15)- Covalent Bonding(14)- More concepts in hybridization theory(1).

 

After studying all types of hybridizations, we now delve deeper into this theory and learn various  cardinal concepts related to it in detail. It is essential that these concepts must be understood clearly to have a compete understanding of this theory.. Let us start studying those concepts first.

  1. % s and p character
    We have already studied that when atomic orbitals mix to form hybrid orbitals , they superimpose on each other in various degrees. Their contribution in making the hybrid orbital can be measured in terms of % character of that atomic orbital in the new hybrid orbital.
    e.g.– 1)In sp3 hybrid orbital , there is one s and three p orbitals. So,out of the 4 orbitals formed , 1 part is contributed by s orbital i.e 1/4th or 25% contribution from s orbital.


∴% s character in sp3 hybrid orbital = 25%

Similarly, 3 parts are contributed by p orbitals i.e 3/4 or 75% contribution from p orbitals.

∴% p character in sp3 hybrid orbital = 75%

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Thus, we can find out % character for any kind of hybridization. The following table gives % character for various hybridizations – 

Type of hybridization

Total # of hybrid orbitals

# of s orbitals

% s character

# of p orbits

% p character

        sp3

4

1

25%

(1 out of 4)

3

75%

(3 out of 4)

       sp2

3

1

33%

(1 out of three)

2

67%

(2 out of 3)

       sp   

2

1

50%

(1 out of 2)

1

50%

(1 out of 2 )

The more s-character a bond has, the stronger and shorter the bond is.

 

685.jpg

This is because, s- orbital is near the nucleus and thus it is strongly attracted by the nucleus(coulombic forces of attraction are high , which stabilize the bond).So, a hybrid orbital with more s- character is strongly bound i.e difficult to break a bond that it forms .It is shorter owing to the fact that it is very strongly attracted to the nucleus.

 

sp > sp2>  sp3 – Strength of the bond.
sp bond is the strongest as it has 50% s – character.

2.Electropositive and electronegative groups – We have already studied that electronegativity is the ability to attract electrons(Refer post # 46).

Electropositivity is the tendency to lose electrons. Generally ,elements who can attain noble gas configuration(octet) by losing electrons are electropositive .e.g. Sodium (11) 1s2 2s2 2p6 3s1. By loosing an electron from 3s orbital, sodium attains octet stability (2s and 2p electrons put together equal to 8).So, sodium is an electropositive element , which has a tendency to lose electrons.

3. Interorbital angle(θ) – This is the angle between two hybrid orbitals.So, ideally ,

     θsp3 = 109.5 º
θsp2 = 120 º
θsp   = 180 º

4.Hybridization Index(i) – For any sphybrid system , i+1 hybrid orbitals are formed.

The ratio of p/s = i , where,

i ⇒ Hybridization index.
p ⇒ no. of p orbitals
s ⇒ no. of s orbitals

For sp3 system, i= 3. So, no.of orbitals formed = i+1=3+1 = 4.

For spsystem ,p/s = 2/1 = 2. So, i=2 and no of orbitals formed = 2+1 = 3 .

For sp system ,p/s = 1/1=1 .Thus, as i= 1, the no of sp orbitals formed are two.

The hybridization index (i) i can also be defined as, 

i= fp / f, where,
fp 
⇒ fraction of p character.
fs ⇒ fraction of s character.

For sp3 system, where 4 orbitals are formed,

fraction of p- orbitalfp= 0.75 (i.e 75%)
fraction of s- orbitalfs= 0.25 (i.e 25%)

The hybridization index i= fp/fs= 0.75/0.25=3.

So basically, % s and fs  are one and the same thing , just in different forms.

In sp3 system, fs= 0.25 or % s character = 25%  and
                          fp= 0.75 or % p character = 75%

Understanding this concept is a little difficult. So, we shall dedicate our next post to understanding this concept fully. Till then, 

Be a perpetual student of life and keep learning …

Good day !

 

 

References and Further Reading –

  1. https://en.wikipedia.org/wiki/Bent%27s_rule#Examples
  2. https://www.quora.com/Inorganic-Chemistry-What-is-the-logic-behind-the-formula-cos-theta-frac-s-s-1-text-or-cos-theta-frac-p-1-p

 

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