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Chemical Bonding

Dr. V.M. Williamson

Department of Chemistry Texas A& M University

Student Version

Valence Electrons●  Electrons in the _____________________●  Determine physical and ______ __________

properties

Na: [Ne]3s1 1s22s22p63s1

3s1

Cl: [Ne]3s23p5 1s22s22p63s23p5

Ti: [Ar]3d24s2 1s22s22p63s23p63d24s2 2

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Formation of Ions = Transfer of e-

!  Note: both ions have a noble gas electron configuration (____ and ____ filled)

!  Stability also with a _________or ________ sublevel

!  How would lithium and fluorine form a compound? 3

Formation of Ionic Compounds!  The underlying reason for the formation of

LiF lies in the electron configurations of Li and F.

1s 2s 2p Li ↑↓ ↑

F ↑↓ ↑↓ ↑↓↑↓↑ These atoms form ions with these configurations.

Li+ ↑↓ same configuration as_____

F- ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ same configuration as ___

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Formation of Ionic Compounds!  Coulomb’s Law describes the attraction

of positive ions for negative ions due to the opposite charges.

( )( )

ions ofcenter between distance dionson charge of magnitude q

ionsbetween attraction of force Fwhere

d

qqF 2

=

=

=

∝−+

• Ion charge ___, Force __ • Distance ___, Force down

• _______ distance, _____ charge gives most force

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Bonding !  Nonmetals= _________ EN = gain e-

!  Metals = ________ EN = lose e-

!  Both form ions and bond by electrostatic attraction; ______ bond

!  Difference in EN of metal and nonmetal will be ______ indicating ________ bondingNa-Cl = 3.0-1.0 = 2.0

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!  When EN difference is _____, no ions are formed.

!  Atoms ‘pull’ about the same; electrons are ______, _______ bond.

!  Consider a C-H bond = 2.5 - 2.1 = 0.4 (in for example CH4)

Formation of Covalent Bonds!  Representation of

the formation of an H2 molecule from H atoms. Covalent Bond by sharing of electrons.

+H HHH

1s1 1s1

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Ionic and Covalent Bonding

Ionic Bonding

Covalent Bonding

Copyright © 1995 by Saunders College Publishing9

Ionic vs Covalent BondingIonic●  Occurs when ΔEN

between bonding atoms is ______

●  _______to nonmetal●  ________ electricity

in the molten state and in solution

Covalent●  Occurs when ΔEN

between atoms is _____________

●  ______to nonmetal●  ____________ in

molten state; poor or non-conducting in solution

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!  Which would you expect to be bonded with ionic bonds?

(A) _____ (B) CO (C) _____ (D) _______

!  The largest difference in electronegativity would be in a bond between:

(A) ______(B) ______(C) N-F (D) ______

The Octet Rule●  When atoms bond they usually lose,

gain or share electrons to attain a noble gas configuration, i.e., usually, a filled outer shell of eight electrons

●  Exceptions are:●  H to B (H, He, Li, Be, B) who have

_________________________________● 3rd period and down elements who can

have ____ than 8 due to possible ‘d’ orbitals

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Ion Formation!  Why S can have oxidation numbers of 2-, 4+,

or 6+??S = 1s22s22p63s23p4 = [Ne] 3s23p4

3s 3pS ↑↓ ↑↓↑ ↑

!  The 2- ion is ___________to fill the ________, !  The 4+ ion is ___________such that the ____

orbital is last, !  The 6+ ion is __________to have a _________

configuration.13

Lewis Dot Formulas: Atoms●  Bookkeeping of valence electrons ●  Each valence electron represented by a dot●  Number of valence electrons = Group number

using A and B notation ●  ________ atoms show electron configuration in

dots

Li Be B C N O F Ne.... .. ..

..HeH

.

.. . .

.. ....

...

..

.. .. .. .... ..

.

Gilbert N. Lewis (1875-1946)!  Berkeley !  Lewis dot structures to

help students understand!  Chemical bond = 1 pair (2)

electrons!  Ionic and covalent bonds!  Octet rule!  Lewis Acid-Base!  Isolated ‘heavy water’!  His student got NP for

isolating deturium Elements that are in the same periodic group have the same Lewis dot structures. 16

Formation of Ionic Compounds!  We can also use Lewis dot formulas to

represent the neutral atoms and the ions they form.

!  Easier than orbital diagrams or electron configuration

+Na Cl

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Understanding Ionic Bonding

2Li + ! 2 [Li] +

1s 2s 2p

Li Li O

1s 2s 2p

Li+

Li+ O2–

O

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Lewis Formula Rules1. Count the valence electrons. 2. Draw the structure with all single bonds and

the octet satisfied. 3. Count the drawing electrons.4. Compare the drawing electrons to the valence

electrons.5. Adjust the drawing if necessary.

"  If there are too many drawing electrons, make multiple bonds.  

"  If there are too few drawing electrons, the octet rule is broken, usually by adding the extra electrons to the central atom. 19

1. Count the valence electrons.!  Find the sum of

valence electrons of all atoms in the polyatomic ion or molecule."  If it is an anion, ____

one electron for each negative charge.

"  If it is a cation, __________ one electron for each positive charge.

PCl3

PCl3

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2. Draw the structure with all single bonds and the octet satisfied. 

!  The central atom is the _______ electronegative element that isn’t hydrogen. Connect the outer atoms to it by _______bonds.

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Writing Lewis Structures-step 2!  Fill the octets of

the outer atoms.

!  Fill the octet of the central atom

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3. Count the drawing electrons.

(__ unshared/1 chlorine) x 3 chlorine= 18

__ unshared on the P__ used in bonding

TOTAL = ____drawing e-

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4. Compare the drawing electrons to the valence electrons.!  Valence of ____!  Drawing of ___!  FINISHED!!

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If there are too many drawing electrons, make multiple bonds. !  HCN has ___ valence electrons!  C is least electronegative!  Satisfying the octet gives this drawing

with ____ electrons, which is NOT correct

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Multiple Bonds●  A covalent bond where atoms share more

than one electron pair●  Double bond: ____________shared

●  Triple bond: _____________shared

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Too Many in the Drawing!  HCN has ___ val. e-!  Octet drawing has ___

e-!  Adjust the drawing to

form multiple bonds!  NOW drawing has ___

electrons, still to many, so make another multiple bond

!  NOW drawing has ___ e- = 10 valence e-, so this is the drawing!!

• •

• •

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If there are too few drawing electrons !  The octet rule is

broken, usually by adding the extra electrons to the central atom.

!  XeF2 has _____ valence e-

!  Octet drawing has ____ e-, too few, so put a pair on Xe

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More Than Eight Electrons

!  The only way PCl5 can exist is if phosphorus has 10 electrons around it.

!  It is allowed to expand the octet of atoms on the 3rd row or below." Presumably __orbitals

in these atoms participate in bonding.

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If the octet rule is satisfied, the number of lone pairs on the X atom is _______ in which pair of structures?

(A) –X , =X (B) =X– , >X–(C) –X–, =X (D) =X=, >X<

If the octet rule is satisfied, the number of lone pairs on the X atom is ________in which pair of structures?

(A) –X , =X (B) ΞX– , –X<(C) ΞX, =X (D) ΞX–, =X<

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Importance of Lewis Structures

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Formal Charges●  Used to select plausible Lewis formula●  Most energetically favorable Lewis formula

for a molecule is when:✻ formal charge on each atom is zero or

closest to zero✻ any negative formal charge is on more

electronegative element✻ formal charges of adjacent or bonded

atoms are not of the same sign

-1 +1 0 0 +1 -1 -2 +1 +1 33

Determination of Formal Charges●  Sum of formal charges (FC) in a neutral

molecule is _________●  Sum of formal charges in a polyatomic ion

is equal to _____________________●  Formal charge on an atom in the

compound from the Lewis formula is:

FC =

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Find the Lewis Structure for CO2!  Val e- = _____

!  Drawing with octet satisfied

!  Drawing has too many (____ e-)

!  Make a multiple bond

!  Drawing still has too many (___ e-)!  So we need another multiple bond, but

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Which?? Find formal Charges!

Val e- – # of bonds – # of lone e- Formal Charge

So, the ____drawing is the best!! It is also __________ 36

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Lewis Dot Formula: BF3!  Valence electrons = ___!  But octet drawing has

___ electrons!  Putting in a multiple

bond gives

Lewis Dot Formula: BF3

!  Consider BF3:" Giving boron a _________places a negative

charge on the boron and a positive charge on fluorine.

"  This would not be an accurate picture of the distribution of electrons in BF3.

Fewer Than Eight Electrons!  Elements from ___

______ are exceptions

!  They usually have the number of bonds = to their ________________.

Copyright © 1995 by Saunders College Publishing

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Carbon monosulfide (CS) was discovered in interstellar space in 1971. With 4 + 6 = 10 available valence electrons, octets are achieved on both atoms with the following structure.

:C Ξ S:What is the formal charge on the Carbon?

(A) __ (B) __ (C) __ (D) __

What is the formal charge on the Sulfur?(A) __ (B) __ (C) __ (D) __

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Bond Order●  Bond order = 1

a ________ bond

●  Bond order = 2 a ________ bond

●  Bond order = 3 a _______ bond

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Copyright © 1995 by Saunders College Publishing

Bond Length●  Distance between

nuclei of bonded atoms

●  Variations in length occur depending on atom neighbors

●  Decreases with increasing bond order

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Selected Bond Lengths

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1 Angstrom = 1 e-10 m OR 1 e10 Angstroms = 1m Another unit to learn

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Resonance

Carbonate ion 45

A Consequence of Resonance

●  Bonding electrons are delocalized●  C-O bond lengths are _____ ______ and

_____________ between a single and a double bond

C

O

O O

2-

How many resonance structures are needed to describe the structure below ?(A) __ (B) __ (C) __ (D) __

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Electronegativities

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EN Difference causes unequal distribution of electrons

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Electron Density Maps!  Shown below is an electron density map of

HF." Blue areas indicate _____electron density." Red areas indicate _____ electron density.

!  It shows a separation of centers of negative and positive charge, an asymmetric charge distribution. Uneven pull on electrons

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Polar vs Nonpolar Covalent Bonds

●  Nonpolar covalent: electron pair(s) ___________between two nuclei of same electronegativity

●  Polar covalent: electron pair(s) ___________between two nuclei of different electronegativities

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Dipole Moment●  A measure of the polarity of a molecule●  Defined as the product of the magnitude of

the partial charges δ + and δ – and the distance d separating these charges

●  Represented by symbol µ in unit of debye (D)

H Cl δ+ δ- dipoles

µ = 1.07 D ∴ polar molecule

dipole moment

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Electron Distribution in Ionic and Covalent Bonds

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What is structure for ICl4- ?

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(A) __ lone pairs (B) __ lone pair (C) __ lone pairs (D) __ lone pairs

What is structure for SO32- ?