Q84 - Chemistry: Molecular Nature Of Matter And Change

Question

Draw a Lewis structure for 

1. The cyclic silicate ion ${\mathbf{Si}}_{\mathbf{6}}{{\mathbf{O}}_{\mathbf{18}}}^{\mathbf{-}\mathbf{12}}$   
2. A cyclic hydrocarbon with formula ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}$  

Step-by-Step Solution

Answer

A Lewis structure represents the bonding atoms, the number of bonds in molecule and the lone pairs. While drawing the Lewis structures, only the valence structures are considered.

Introduction

Lewis structure are the electronic structure where the valence electrons around individual atoms in a molecules are shown. A Lewis structure represents the electrons as dots, and the bonding electrons are depicted as a line between two atoms.

Steps to write the Lewis structures

The steps to write the Lewis structure-

• First the total number of valence electrons is calculated adding the individual valencies of individual atom.
• In case of anion, extra electrons are added to the Lewis dot structure.
• In case of cations, electrons are subtracted from the total number of electrons in order to make up for the positive charge.
• The central atom is the least electronegative atom of the molecule or ion.
• The atoms are connected by single bonds

 

• Now, each atom belonging to the molecule, is assigned the lone pairs and lone pairs are first assigned to the most electronegative atom.

 

•  Now, put the remaining electrons about the central atom.

 

To write the Lewis structures of Si 6 O 18 - 12

Silicon is a nonmetal having 4 valence electrons. It shows the property of catenation and so is capable of forming extended networks.

In ${\mathrm{Si}}_6{{\mathrm{O}}_{18}}^{-12}$, there are 6 Si atoms and 18 O atoms. Si has four valence electrons, each electron is used up in bonding with oxygen on all the four sides, so no lone pair is left on silicon. Each oxygen has 6 valence electrons of it and an extra electron since it is an anion. Terminal oxygen is bonded to Si on one side, so have in total 3 lone pairs of electrons, while the oxygen atoms towards center are each bonded to Si on two sides (so 2 electrons are used up in bonding); thus, two lone pairs of electrons are left on this oxygen as shown in the figure.

To write the Lewis structure of a cyclic hydrocarbon with formula C 6 H 12

A cyclic hydrocarbon with formula ${\mathrm{C}}_6{\mathrm{H}}_{12}$ is cyclohexane. In cyclohexane 6 carbon atoms and 12 hydrogen atoms are there, each carbon having 4 valence electrons, is bonded to carbon atom from two sides (so 2 electrons of carbon are used up in bonding), while the remaining two electrons are each used up in bonding with two hydrogen atoms. So, there is no lone pair left in the molecule, as each is used in bonding.