Question

Why in inductive effect slightly shifting of sigma bond takes place?

Answer 1

In inductive effect there is no actual shifting of sigma bond, Only the electron cloud moves towards a specific atom along the sigma bond.

Inductive effect occur due to the effect of electronegetivity. The more electronegetive atom pulls the electron cloud more towards itself hence attaining a partial negative charge, and the less electronegetive atom gets partially positively charged. 

Inductive effect is only effective to about 2-3 atoms, after which it becomes negligible 
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+ve Inductive effect:
When the electron cloud is pulled along the sigma bond it is +ve inductive effect. Generally shown by electronegetive atom (like halogen).

-ve Inductive effect
When the electron cloud is pushed along the sigma bond it is -ve inductive effect. Generally shown by electropositive atom (like metallic elements).

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ELECTRONEGATIVITY ORDER:
F>Cl>Br>I>S>C>H>P

Inductive effect is a permanent effect .