第二章第二章第二章第二章

Chemical Bonds

2-2 Lewis Structure

Lewis Symbols for Atoms

Source from San Diego Miramar College

Covalent Bond

H H H H+

Ionic Bond

Octet Rule

建立共價鍵分子步驟建立共價鍵分子步驟建立共價鍵分子步驟建立共價鍵分子步驟

� Valence Electrons

� Count the total valence electrons for the molecule

� Octet Electrons

� Figure out how many octet electrons the molecule should have, using the octet rule

� Bonding Electrons

� Subtract the valence electrons from octet electrons

� Total Number of Bonds

� Divide the number of bonding electrons by two

� Draw a Skeleton Structure

� Place Chemical Bonds

� Assign each connection with one bond, the extra bonds will go to double bond or triple

Bond

� Place Lone Pairs

� Assign the extra valence electrons as the lone pairs on each atoms following the octet

rule

共價鍵共價鍵共價鍵共價鍵 Lewis Structure

How to Arrange Atoms

� Identify Terminal Atoms: H, F, Cl, Br, I

� Identify Terminal Groups: CH3

� Pick Central Atom

� Atom with the lowest IE

� Arrange the atoms symmetrically around the

central atoms

Example 1

� Draw the Lewis structure of hydrogen cyanide HCN

� Valence Electrons

� Octet Electrons

� Bonding Electrons

� Total Number of Bonds

� Draw a Skeleton Structure

� Place Chemical Bonds

� Place Lone Pairs

Example 2

� Draw the Lewis structure of thionyl chloride SOCl2� Valence Electrons

� Octet Electrons

� Bonding Electrons

� Total Number of Bonds

� Draw a Skeleton Structure

� Place Chemical Bonds

� Place Lone Pairs

離子鍵離子鍵離子鍵離子鍵 Lewis Structure

Formal Charge (FC)

� The formal charge on an atom in the Lewis

structure is the charge it would have, if the

bonding were perfectly covalent.

� FC = V – (L+ S/2)

� V : the number of valence electrons for the molecule

� L : the number of the lone-pair electrons

� S : the number of the shared electrons

Examples of CO2 and N2O

The Most Favorable Arrangement

Example of (SCN)-

� Write 3 different possible Lewis structure of

(SCN)- and calculate the formal charges, then

identify the most favorable arrangement of

atom.

Example of (CH3NHO)-

� Write 2 different possible Lewis structure of

(CH3NHO)- and calculate the formal charges,

then identify the most favorable arrangement

of atom.

Example of (NO3)-

� Write 3 different possible Lewis structure of

(NO3)- and calculate the formal charges, then

identify the most favorable arrangement of

atom.

Resonance

Resonance Structure of C6H6

Structure Evidence

Example

� Write two Lewis structures of O3 that

contribute to its resonance structure.