How electronegativity depends on oxidation number-hybridization and what is group electronegativity ?

How electronegativity depends on oxidation number,hybridization and other factors?

Although the electronegativity of an element is conventionally assigned a definite value, yet it may differ from one molecule to another.

The ability of an atom in a molecule  to attracts electron towards itself depends upon its environment in the molecule.

The electronegativity of an atom would thus depend upon the nature of the other atom with which it is attached in the molecule.

Besides the atomic size , the  factors which largely determine the elctronegativity  of an atom are  hybridization , oxidation state, substituents, ionization energy and effective nuclear charge.

Charge on the atom 

An atom which acquires a positive charge would tend to attract electrongs more strongly than a neutral atom.

Thus, a cation will be more electronegative than the parent atom. But an anion would be less electronegative than the neutral atom.

Why HClO₃ behaves as a stronger acid than HClO?

The  electronegativity of an atom depends on  its oxidation state of the concern element.The oxidation state of central chlorine atom is +1 in HClO acid and +5 in HClO₃ acid.Hence, the Cl atom in HClO₃ acid will be more electronegative than  in HClO acid.

Accordingly, the release of hydrogen as H+ ion in HClO₃ would be much easier than a similar change in HClO. In other words , HClO₃ behaves as a stronger acidthan HClO.

Hybridization 

We know that ‘s’ electrons are more penetrating than the ‘p’ electrons. 

Therefore, if an atom has hybrid orbitals with a greater ‘s’ character, the electronic charge in such orbitals would remain closer to the nucleus of that atom.

This means that the electronegativity of the atom would increase with increase in ‘s’ character of its hybrid orbitals.

Why acetylene is more acidic than ethylene and methane?

The hybridized states  of carbons in  acetylene, ethylene and methane are  sp , sp2 , sp3 respectively. As the ‘s’ character changes , the eletronegativity of the carbon  atom changes.  

With increase in the percentage of  ‘s’ character  electronegativity of  the atom increases. 
Hence, the order of electronegativity is  C (sp) > C ( sp2) > C ( sp3) .

Consequently, acetylene is more acidic than the other two, and the acidity order is as follows.

Why alkyl cyanide is less basic than  pyridine and aniline?

 The hybridized states  of nitrogen atoms  in alkyl cyanide, pyridine and aniline are  sp , sp2 , sp3 respectively.

As the ‘s’ character changes , the eletronegativity of the  nitrogen  atom changes.  With increasing  the percentage of  ‘s’ character  electronegativity of  the atom increases.

Hence, the order of electronegativity is  N( sp) > N( sp2) > N( sp3). Hence , the availability of electron for protonation is highest  for aniline and lowest for alkyl cyanide.

Consequently, aniline is a stronger base than pyridine  which, in turn, is a stronger base than alkyl cyanide.

Effect of substituents

The electronegativity of an atom depends on the nature of the substituents attached to that atom.

For example , the carbon atom in CF₃I acquires a greater positive charge than in CH₃I . Hence, the carbon atom in CF₃I is more electronegative than CH₃I .

Role ionization energy and electron affinity 

According to Mulliken ‘s equation of electronegativity, the higher the ionization energy and electron affinity , the greater would be the electronegativity of the atom.

Effective nuclear charge and electronegativity 

Generally, the electronegativity of elements  are proportional  to the  Zeff. of the elements.

What is group electronegativity ?

Some times we meet with groups such as  - NO₂  ,  - CN , -Me etc and it is advantageous to consider the electronegativity of the group as a whole rather than calculating those for indivisual elements.

These have been termed as group electronegativity.

For example,  Let us consider  - CH₃ and  - CF₃ .Although  in both cases we are considering the bond of carbon, the carbon atoms are not definitely identical.

In – CF_3, the carbon atom acquires considerable positive charge and therefore must possesses higher electronegativity than the carbon in –CH₃ .

As a general rule the order  of inductive effects will be the order of group electronegativity, that is , the group with higher  - Ieffect will have higher group electronegativity.

Use of electronegativity values:Amount of ionic character.

The amount of ionic character is proportional to the difference in electronegativity.

If two atoms  A and B possess same electronegativity values then bond  A – B  will be purely covalent.

However, if their be the difference in electronegativity  it will lead to ionic character. 

Pauling plotted  the difference in electronegativity with  the percentage of ionic character ( which obtained  from dipole moment measurement ) and obtained the following curve.

Thus, we can use the difference in electronegativity to predict the type of bonding.

Summary

• How electronegativity depends on hybridization?
• Why acetylene is more acidic than ethylene ?
• Why acetylene is more acidic than methane ?
• Why chloric acid is stronger than hypochlorous acid ?
• What is group electronegativity ?
• How electronegativity depends oxidation number?

Click here: What is electrnegativity ?