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Chemical Bonding What holds things together?. PGCC CHM 101 Sinex. Conductivity - high. Conductivity - low. Let’s examine the melting point of compounds across two periods. What is the trend?. high. low. CRC Handbook of Chemistry and Physics , 1995. Bonding.

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slide2

Conductivity - high

Conductivity - low

Let’s examine the melting point of compounds across two periods.

What is the trend?

high

low

CRC Handbook of Chemistry and Physics, 1995

bonding
Bonding

Can we explain the melting point behavior across a period?

  • involves the valence electrons or outermost shell (or highest shell) electrons
  • for group A elements - the group number tells how many valence electrons

How many valence electrons on N?

Group 5A – 5 valence electrons

bonding1

F

F

Bonding…
  • Lewis dot structures show the valence electrons around at atom and for most molecules and compounds a complete octet for the elements
  • most monatomic ions have an electron configuration of noble gases

Al

N

1s22s22p5

+ e-

1s22s22p6 Ne

write out the electron configuration for the following atoms and ions
Write out the electron configuration for the following atoms and ions:

He

Ne

Ar

Kr

H-

Ca+2

O-2

Br-

Which noble gas is isoelectronic with each ion?

slide6

Cl

Cl

NaCl

This is the formation of an ionic bond.

-

+

Na

Cl

electron transfer

and the formation of ions

Cl2

This is the formation of a covalent bond.

sharing of a pair of electrons

and the formation of molecules

what about the distance between the atoms in a bond
What about the distance between the atoms in a bond?

NaCl Na+ Cl- d = 281 pm

Cl2 Cl-Cl d = 199 pm

What property can be used to tell when a bond will ionic or covalent?

slide10

Electronegativity

The electronegativity difference - DEN = ENhigher – EN lower

large difference

small difference

slide11

Using electronegativities to determine bond type

DEN > 1.7 ionic bond - transfer

DEN < 1.7 covalent bond - sharing

So we have a range of electronegativity difference of 0 to 1.7 for sharing an electron pair.

slide12

X

Y

DEN = 0

X

Y

DEN = 0.3

X

Y

DEN = 0.6

increasing polarity of bond

X

Y

DEN = 0.9

polar bond

0 < EN< 1.7

X

Y

DEN = 1.2

Is the sharing of electrons in molecules always equal?

non-polar bond

Which element is more electronegative?

ENY > ENX

Direction of electron migration

slide13

Space-filled

BF3 – a planar molecule

F

4.0

B

2.0

Ball & stick

negative

top

side

positive

Electrostatic potential maps

Spartan ‘02

slide14

O

O

O

O

O

O

double bond (2 pairs)

N

N

N

N

N

N

N

N

triple bond (3 pairs)

More sharing examples

O2

Share until octet is complete.

N2

octet complete

bond energy

increasing bond strength

Bond Energy

Is breaking a bond an endothermic or exothermic process?

F2 single bond BE = 142 kJ/mole

O2 double bond BE = 494

N2 triple bond BE = 942

X2 + energy  X + X

http://wulfenite.fandm.edu/Data%20/Table_6.html

slide16

normal covalent bond

(each atom supplies an electron)

coordinate covalentbond

(the pair of electrons from the same atom)

H

H

H

H

H

H

Some more sharing examples

NH3

N

NH4+

NH3 + H+ NH4+

H+

N

slide17

Type of bond? – I, PC, or NC

TiO2

CH4

NaI

CS2

CO2

KCl

AlCl3

CsF

HBr

slide18

Using the EN trends to predict bond type

Increasing EN

Increasing EN

105

Db

107

Bh

NO

RbF

FeS

H2S

Modified from http://www.cem.msu.edu/~djm/cem384/ptable.html

slide19

Draw the Lewis dot structures

CO2

NH2-

H3O+

CO

HCN

H2CO

(C in center)

show the direction of electron migration in the following
Show the direction of electron migration ( ) in the following.

C – H

H – F

C = O

C – Cl

Rank the bond polarity (1-most … 3-least)

As-H N-H P-H

slide21

Here is the electrostatic potential map for H2CO.

Show the electron migration on this planar molecule.

H

C O

H

blue – positivered - negative

How is this molecule different than BF3?

slide22

Comparison of Bonding Types

ionic

covalent

ions

molecules

molten salts conductive

non-

conductive

valence electrons

transfer of electrons

sharing of electrons

high mp

low mp

DEN > 1.7

DEN < 1.7

slide23

A B

A+ B-

A B

Increasing DEN

Increasing polarity

100% covalent

100% ionic

Bonding spectrum

Transfer