chapter 8: basic concepts of chemical bonding

Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding

Chemical Bonds

• ionic bond

• covalent bond

• metallic bond

electrons are transferred from oneatom to another atom

electrons are shared between two atoms

electrons are free to move between all atoms


The formation of chemical bonds involves valence electrons

Lewis Symbols: “shorthand” for showing valence electrons

Gilbert Lewis (1875-1946)


Lewis dot structures:

H

Li

He

Be B C N O F Ne

Na Mg Al Si P S Cl Ar

For representative (main group) elements:group number = number of valence electrons

1A 2A 3A 4A 5A 6A 7A 8A


Noble gases, except helium, have 8 valence electrons

ns2np6

When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence

electrons (an “octet” of electrons)

K Cl+ K+ + Cl

[Ar] [Ar]electron configuration:


Octet Rule:

Na+ Cl-

Cl- Na+

Na+ Cl-

Formation of an ionic lattice


Na+ (s) + Cl- (g) NaCl (s) very exothermicH<0


dQQ

Eel

charge on ions

distance between charges

Strength of ionic bond depends on Eel

• the larger Eel, the stronger the bond

• the greater the charges, the stronger the bond

• the smaller the distance between the charges, the stronger the bond


The stronger the ionic bond the the melting pointhigher

SrI2 +2, -1

66, 133

66, 140

113, 220

1261oC

2852oC

538°C

r1 r2


Chapter 8: Basic Concepts of Chemical BondingcChapter 8: Basic Concepts of Chemical Bondingc

Covalent Bonds …

… are formed when two atoms share electrons

… in order to achieve “noble gas configuration”

H H+ H H Each hydrogen has the electron configuration of He

F F+ F F Each fluorine has the electron configuration of Ne

… of the nearest noble gas


Covalent Bonds

H H+ H H

F F+ F F

H H

F F

• A shared electron pair is drawn as a dash (two electrons!)

• Unshared electrons are drawn as dots


Single and Multiple Bonds

F F+ F FF F or

O O+ C + O C O O C Oor

N NN + N N Nor

Single bond

Double bond

Triple bond


Single and Multiple Bonds

X X

X X

X X

Distance between atoms (bond

length) decreases

Bond strength increases


Bond Polarity and Electronegativity

Cl Cl non-polar covalent bond:equal sharing of electrons

• When both atoms attract bond electrons equally, electrons are shared equally



H Cl polar covalent bond:unequal sharing of electrons

+

• If one of the atoms attracts bond electrons more strongly, electrons are shared unequally



H Cl polar covalent bond:unequal sharing of electrons

+

For comparison:

ionic bond:electrons are not sharedNa+ Cl -


How do we know when a non-polar, polar, or ionic bond is formed?

The tendency of atom to attract electrons when forming a molecule is

summarized in the concept of

electronegativity

Incr

ease

in E

lectro

nega

tivity


How do we know when a non-polar, polar, or ionic bond is formed?

If the difference in Electronegativity, EN, is…

…smaller than 0.5EN < 0.5 => nonpolar bond

…greater than or equal to 0.5 AND smaller than 2.00.5 EN < 2.0 => polar bond

…greater than or equal to 2.0EN 2.0 => ionic bond


Among the following examples, which bond is most polar?

S

all equally non-polar

C-F (C-H is non-polar)

P-Cl (furthest apart in P.T. => largest EN)

all equally non-polar

e) O-I O-N O-F O-I (furthest apart in P.T. => largest EN)


Among the following examples, which bond is shortest?

S

H-H

C-H

Cl-Cl

C C

Bond length depends on (a) radii of the bonded atoms

(b) the number of bonds between atoms

remember that the atomic radii decrease along a period in the P.T.


Drawing Lewis Structures of Moleculesthere are no Lewis structures for ionic compounds!!

If the compound contains more than 2 atoms:

• how are the atoms bonded and,

• if there are nonbonding electron, where are they?


Molecules with a central atom : NH3, PCl3, CHCl3

central atom is generally the first in the molecular formula

NH H

HC

H

Cl Cl

Cl

PCl

Cl

Cl


…unless the first element is Hydrogen :

H has only one valence electron => can only make one bond

H2OO

H H

HCN C NH

(same order as in formula)


(1) sum valence electrons from all atoms: these are the ones that need to be distributed

(3) complete "octets" of atoms bound tocentral atom

(4) place any leftovers from (1) on the central atomcheck that central atom has octet

(2) connect atoms by covalent bonds: count electrons used up

(5) If there are not enough electrons to give the central atom an octet, try multiple bonds

NH3

8

NH H

H

6

n/a

+ 2

n/a

Rules for Drawing Lewis Structures


(1) sum valence electrons from all atoms: these are the ones that need to be distributed

(3) complete "octets" of atoms bound tocentral atom

(2) connect atoms by covalent bonds: count electrons used up

(5) If there are not enough electrons to give the central atom an octet, try multiple bonds

CO10

2

+ 6

+ 2

triple bond!

C O

C O

(4) place any leftovers from (1) on the central atomcheck that central atom has octet

chapter 8: basic concepts of chemical bonding

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