Chemistry 101 : Chap. 8 Basic Concepts of Chemical Bonding (1) Chemical Bonds, Lewis Symbols and the Octet Rule (2) Ionic Bonding (3) Covalent Bonding

Chemistry 101 : Chap. 8Chemistry 101 : Chap. 8

Basic Concepts of Chemical Bonding

(1) Chemical Bonds, Lewis Symbols and the Octet Rule

(2) Ionic Bonding

(3) Covalent Bonding

(4) Bond Polarity and Electronegativity

(5) Drawing Lewis Structures

(6) Resonance Structures

(7) Exceptions to the Octet Rule

(8) Strengths of Covalent Bonds

Chemical BondsChemical Bonds

Chemical bond is formed when two atoms or ions are held

together by the attractive force between them.

Ionic Bond : a chemical bond formed between cation and anion

Covalent Bond : a chemical bond formed between two nonmetallic

atoms by sharing one or more pairs of electrons.

Metallic Bond : a chemical bond formed when valence electrons

of metal atom are attracted by the nuclei of surrounding atoms

(electrons are free to move throughout the metal)

Lewis SymbolsLewis Symbols

Lewis electron dot structure (or Lewis symbol) : Symbol of

element surrounded by dots representing the valence electrons

in the atom

Gilbert N. Lewis (1875-1946)

Lewis symbol for sulfur : [Ne]3s23p4

S

Maximum 2 electronson each side

This works only for representative

elements (main group)


Elements Group e- Configuration Lewis Symbol

Hydrogen 1A 1s1 H

Helium 8A 1s2 He

Lithium 1A [He]2s1 Li

Berylium 2A [He]2s2 Be

Boron 3A [He]2s22p1 B

Carbon 4A [He]2s22p2 C


Elements Group e- Configuration Lewis Symbol

Nitrogen 5A [He]2s22p3 N

Oxygen 6A [He]2s22p4 O

Fluorine 7A [He]2s22p5 F

Neon 8A [He]2s22p6 Ne

Note : All four sides of the symbol are equivalent

O O=


Elements in the same group of periodic table have the same

Lewis symbols

F Cl Br I

Elements in the same group have the same valence electron

configurations

For halogen atoms : ns2np5

Octet RuleOctet Rule

Only the valence electrons are involved in chemical bonding.

Octet Rule : When forming chemical bond, atoms tend to gain,

loose or share electrons in order to achieve a complete octet of

valence electrons (ns2np6)

same electron configuration as noble gas atom

K Cl+ K+ + Cl

Both ions have an octet of electrons !

[Ar] [Ar]electron configuration:

Ionic bondingIonic bonding

Ionic Bonding : Cations (metals) and anions (non-metal) combine to form ionic bonds

NaCl

Alternating positive and negativecharges


Metal : small ionization energy

Na(g) Na+(g) + e- IE = 496 kJ

Non-metal : large electron affinity

Cl(g) + e- Cl-(g) EA = -349 kJ

NaCl formation : Na(s) + ½ Cl2(g) NaCl (s) Hof = -490 kJ

Removing an electron from Na and transferring it to Cl is NOT exothermic !

Then, why NaCl formation is an exothermic process?


The main driving force to form ionic bonds is the electrostatic interaction between positive and negative ions.

dQQ

Eel

distance between ions

charges of ions

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

SrF2 +2, -2

66, 133

66, 140

113, 133

1261oC

2852oC

1473oC

r1 r2

Covalent bondingCovalent bonding

Covalent bond is formed when two atoms share electrons in order

to achieve the electron configuration of the nearest noble gas.

satisfy octet rule

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

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

H H+ H H

F F+ F F

H H

F F

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

Unshared electrons are drawn as dots (lone-pair electrons)


Lewis dot structure for covalent bonds

single covalentbond


Example : Draw the Lewis dot structures of H2O and NH3


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

Multiple bond

X X

X X

X X

Distance between atoms (bond

length) decreases

Bond strength increases


Single and Multiple bond

Drawing Lewis StructureDrawing Lewis Structure

Things to know before you start to draw Lewis structure

Chemical formulas are often written in the order in which the atoms

are connected

ex) HCN

Hydrogen has only two electrons (shared) and always has only

one covalent bond

The central atom is usually written first

ex) NH3, CCl4, CHCl3, PCl3


Rules for drawing Lewis structure

(1) sum the number of valence electrons from all atoms

(2) write the symbols for the atoms and connect them with a single bond

(3) complete the "octet rule" for the atoms bonded to central atom

(4) place any left over electrons on the central atom

(5) If there are not enough electrons to give the central atom

8 electrons, try multiple bonds.


Lewis Structure of NH3

(1) Total number of valence electrons = 5 + 3 1 = 8

(2) Connect atoms with a single bond H N H

H and count the number electrons used

for single bond = 6

(3) Complete the octets on the atoms bonded to the central atom : done

(4) Place remaining electrons

(8-6=2) on the central atom

H N H

H

(5) All atoms are satisfying octet. No need to consider multiple bonds


Lewis Structure of CO

(1) Total number of valence electrons = 4 + 6 =10

(2) Connect atoms with a single bond

and count the number electrons used

for single bond = 2

C O

(3) Complete the octets on the atoms bonded to the central atom (6 electrons are used) C O


(10-2-6 = 2) on the central atomC O

(5) Carbon is NOT satisfying octet rule. Need to have multiple bonds C O C O


Example : Determine the Lewis structure of HCN

(1) Total number of valence electrons

(2) Connect atoms with a single bond and count the number electrons used for single bond

(3) Complete the octets on the atoms bonded to the central atom

(4) Place remaining electrons on the central atom.

(5) Carbon is NOT satisfying octet rule. Need to have multiple bonds


Example : Determined the Lewis structure of CH2O





(5) Carbon is NOT satisfying octet rule. Need to have multiple bonds


Example : Determined the Lewis structure of H2O2






What happens if you choose a different geometry in step (2)?


Lewis Structure of ClO3- [ion]

(1) Total number of valence electrons = 7 + 63 + 1 = 26

(2) Connect atoms with a single bond and count the number electrons used for single bond = 6

O Cl O

O

(3) Complete the octets on the atoms bonded to the central atom (18 electrons are used)

O Cl O

O


(26-18-6 = 2) on the central atomO Cl O

O


O Cl O

O

Example : Determined the Lewis structure of ClO2-






on the central atom


Drawing Lewis Structure : Exceptions


Atoms having fewer than 8 valence electrons :

Group IIA and IIIA (mostly Be, B).

Example = BeCl2(1) Total number of valence electrons = 2 + 2 7 = 16


Cl Be Cl


Cl Be Cl

(4) Place remaining electrons (16 - 4 -12 = 0) on the central atom : None left

(5) Be is not satisfying the octet rule, but no electron is available:



Atoms having more than 8 valence electrons :

central atom with n 3, which can use d-orbitals for bonding

Example = SF4



S

F F

F F

(3) Complete the octets on the atoms bonded to the central atom (24 electrons are used) S

F F

F F(4) Place remaining electrons

(34-8-24 = 2) on the central atom

(5) S is not satisfying the octet rule (10 electrons) S

F F

F F



Molecule having an odd number of valence electrons :

Example = NO2



O N O


O N O


(17-4-12 = 1) on the central atomO N O

(5) Nitrogen has only 5 electrons. Need to have multiple bonds O N O Free radical



Example : Determine the Lewis structure of BF3, BrF5 and OH

Drawing Lewis Structure : Resonance


Lewis Structure of SO3


(2) Connect atoms with a single bond and count the number electrons used for single bond = 6 S

O

O O(3) Complete the octets on the atoms bonded to the central atom (18 electrons are used)

S

O

O O(4) Place remaining electrons on the central atom.

No more electron is left (24-6-18=0)

(5) Sulfur is NOT satisfying octet rule. Need to have multiple bonds S

O

O O S

O

O O S

O

O O

Resonance structuresAll three S-O bonds have the same length



Example : Determine the Lewis structure of O3 and HCO2-

Properties of Covalent BondProperties of Covalent Bond

Bond length : The distance between two bonded atoms

bond length

Bond length depends on the size of two atoms and the

number of covalent bond (single, double or triple) between them.


Example : Predict which member of each set would have the shortest bond length

SS


Bond Enthalpy: Energy required to completely separate two

bonded atoms in gas phase. A short bond is usually harder to break.

C

H

H H

H (g)

C (g) + 4 H (g) H = 1660 kJ/mol

C H (g) C (g) + H (g) D (C-H) = 1660/4 kJ/mol = 415 kJ/mol

per C-H bond:


Bond enthalpy can be used to estimate the enthalpy change

of chemical reactions, Hrxn

H2(g) + Cl2(g) 2HCl(g) H = ?

H1 H2

Hrxn


H1 = D(H-H) + D(Cl-Cl) = 436kJ/mol + 243kJ/mol = 679 kJ/mol

H2 = 2 [ D(H-Cl) ] = 2 - 431 kJ/mol = - 862 kJ/mol

Hrxn = H1 + H2 = 697kJ/mol – 862 kJ/mol = -183 kJ/mol

Horxn = Σ n x Dbroken – Σ m x Dformed

moles of bonds

Hrxn


+ +

bonds broken

H – H 1Cl – Cl 1

bonds formed

H – Cl 2

Bond Enthalpy (kJ/mol) H – H 436 Cl – Cl 243 H – Cl 431

Hrxn = 1 436 + 1 243 – 2 431

= - 183 kJ/mol


Example : Estimate the Hrxn of following reaction

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

ElectronegativityElectronegativity

Electronegativity : A measure of the attraction an atom has

for the electron in a bond

Linus Carl Pauling (1901-1994)

Metals low electronegativity

Nonmetals high electronegativity

electronegativity scale:

Fluorine = 4 (most electronegative)

most strongly attracting electron

Cesium = 0.7 (least electronegative)

most easily giving up electron

ElectronegativityElectronegativity

Pauling scale of electronegativity

Element EN

F 4.0

O 3.5

Cl 3.0

N 3.0

C 2.5

H 2.1

Bond PolarityBond Polarity

Nonpolar covalent bond

When two atoms of same element are bonded together,

there is equal sharing of the electrons in the bond

Cl Cl

non-polar covalent bond:equal sharing of electrons

Bond PolarityBond Polarity

Polar covalent bond

H Cl

+

When two different elements are bonded together,

there is unequal sharing of the electrons in the bond

polar covalent bond:unequal sharing of electrons

The bonding pair of electrons

is pulled toward the chlorine

atom (partial charge)

Bond Polarity and ElectronegativityBond Polarity and Electronegativity

H Cl

+

polar covalent bond:unequal sharing of electrons

Na+ Cl -

ionic bond:electrons are not shared

EN = 3.0 – 2.1 = 0.9 EN = 3.0 – 0.9 = 2.1

EN < 0.5

non-polar bond

0.5 EN < 2.0

polar bond

EN 2.0

ionic bond

Bond Polarity and ElectronegativityBond Polarity and Electronegativity

Example : For each pair of bonds, predict which bond is more polar

and the partial charge on the atoms

(a) Cl – Br Br – F

(b) O – F S – F

(c) C – H C – O

(d) H – O Na – O

Documents

Chemistry 101 : Chap. 8 Basic Concepts of Chemical Bonding (1) Chemical Bonds, Lewis Symbols and the Octet Rule (2) Ionic Bonding (3) Covalent Bonding