Q10.13P - Chemistry: Molecular Nature Of Matter And Change

Question

Draw the Lewis structure with the lowest formal charges, and determine the charge of each atom in

(a)  ${\mathbf{IF}}_5$

(b)  ${{\mathbf{AlH}}_4}^{-}$

Step-by-Step Solution

Answer

The Lewis structure of

a.${\mathbf{IF}}_5$ = square pyramidal with central atom I-atom having -1 charge and have zero formal charges.

b. ${{\mathbf{AlH}}_4}^{-}$= tetrahedral shape with central atom Al-atom having +3 charge and negative (-1) formal charge.

 

The lowest formal charge present on ${\mathbf{IF}}_5$ having 0 formal charge.

Lewis Structure

A Lewis structure can be defined as the basic structure of a molecule which used the valence electron of each atom involved. In Lewis’s structure, the least electronegative atom will be the central atom. 

 

Formal Charge is a theoretical concept which is also called fake charge. It is the charge which is dispersed on the whole molecule, not on the single atom.

 

Formal Charge = Valence electron – Lone pair electron – ½ (Bonded electron)

 

Explanation

I and F belong to group 17 having 7 electrons in the valence required to give or take 1 electron to fulfil its octet. In Lewis’s structure, there is bond formation by the electron pair provided by one I-atom and 5 F-atom. As the I-atom can expand its valence because of the presence of a vacant d-orbital and a lone pair left.

                                          

 $\mathbf{Formal}\mathbf{ }\mathbf{charge}\mathbf{ }\mathbf{on}\mathbf{ }\mathbf{I}\mathbf{ }\mathbf{atom}\mathbf{ }\mathbf{of}\mathbf{ }{\mathbf{IF}}_5\mathbf{molecule}\mathbf{=}\left(\mathbf{7}\mathbf{-}\mathbf{2}\mathbf{-}\left(\frac{10}{2}\right)\right)\mathbf{=}\mathbf{0}$

Clarification

Al belongs to group 13 having 3 electrons in the valence and requires to give or take 5 electrons to fulfil its octet, H-atom belongs to group 1 having 1 electron in the valence shell and requires 1 electron to fulfil duplet.  In Lewis’s structure, the Al-atom needs 5 electrons which will be provided by the 4 H-atom and one electron over the whole molecule provided by some metal atom. 

                            

 $\mathbf{Formal}\mathbf{ }\mathbf{charge}\mathbf{ }\mathbf{on}\mathbf{ }\mathbf{Al}\mathbf{ }\mathbf{atom}\mathbf{ }\mathbf{of}\mathbf{ }{\mathbf{AlH}}_{\mathbf{4}}\mathbf{ }\mathbf{molecule}\mathbf{=}\left(\mathbf{3}\mathbf{-}\mathbf{0}\mathbf{-}\left(\frac{4}{2}\right)\right)\mathbf{=}\mathbf{1}$