Chemical Bonding

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Transcript Chemical Bonding

Chemical Bonding

Chemical Bonding

    is all about the

electrons

.

in most of our discussions we will concentrate on the outer ‘s’ and ‘p’ electrons. These are called the

valence electrons.

since there are 4 of these orbitals in any quantum it requires 8 electrons (an octet) to fill them.

the tendency of atoms to try to fill out the outer ‘s’ and ‘p’ shells is the

octet rule.

Chemical Bonding

 Elements will combine in order to fill their valence shell with electrons, like the noble gases. This can happen in one of two ways: 

To share electrons

  The outer orbitals of 2 atoms overlap so that each atom is in the vicinity of a full set of valence electrons.

This type of bonding is called

covalent bonding

.

Chemical bonding

    Atoms

gain or lose electrons

to arrive at a full set of valence electrons.

When atoms gain or lose electrons they become

ions

. Ions are attracted to ions of opposite charge and repelled by ions of the same charge. This type of bonding is called

ionic bonding

How Many Bonds ???

• the number of bonds made by atoms in the ‘s’ and ‘p’ blocks of the Periodic Table is determined by how many electrons they are away from an octet:

How Many Bonds ???

Group Valence Electrons Covalent Bonds Ionic Bonds 1 2 13 14 1 2 3 4

1 2 1 3 1 4 1 1 (1+) 2 (2+) 3 (3+)

15 16 5 6

3 (5 2 ) 2 (6 2 ) 3 (3-) 2 (2-)

17 7

1 (7 2 ) 1 (1-)

18 8

0 (8 2 ) 0

1 electron promotion 2 valence level expansion

• most of our discussion will be centred on covalent bonding.

Valence level expansion

 Some compounds occur which cannot be easily explained: PF 5 , SF 6 , ClF 7 , ArF 8   in each case the number of chemical bonds is equal to the number of valence electrons.

this can only happen if electrons are promoted to a higher energy level. In this case it is the adjacent ‘d’ orbital.

PF

5  the normal orbital diagram looks like this:  with valence level expansion it looks like this:

Valence level expansion

  includes elements of groups 15 to 18, from period 3 down; periods 1 and 2 do not have a ‘d’ orbital to promote to.

Please note that valence level expansion is the

exception

, not the rule.

Polar-Covalent Bonding

  Covalent bonding implies

equal

sharing of electrons.

If sharing is not equal, the electrons in a bond will spend more time with one atom than the other.

Polar-Covalent Bonding

  The atom where the electrons spend more time will have a net

negative

charge, while the atom at the other end of the bond will be

positive

.

This type of bond is

polar-covalent

.

Electronegativity

   is a measure of how strongly an atom is holding on to its valence electrons. If an atom loses an electron fairly easily it has a low electronegativity (and tends to be a cation). If an atom tends not to lose electrons, but tends to steal them from other atoms (and become an anion) it has a high electronegativity.

 To determine what type of bond exists between two atoms you subtract their respective electronegativities:  

if the electronegativity difference is 0.2 or less, the bond is covalent if the electronegativity difference is 1.7 or greater the bond is ionic.

Electronegativity

  If the electronegativity difference between two atoms is between 0.3 and 1.6 the bond is

polar covalent.

The greater the electronegativity difference the greater the ionic character of the bond:

Assignment

 Determine the electronegativity difference for each chemical bond. If the bond is polar covalent draw an arrow in the direction of the dipole, from positive to negative: N - H P - H Br - Cl B - F Si - Cl O - H C - H END = | 2.5 - 2.1 | = 0.4

polar covalent bond S - O Cu - Br C - Cl N - I C - O

N - H • END = |3.0 – 2.1| = 0.9

• polar covalent bond: N - H B - F • END = |2.0 – 4.0| = 2.0

• ionic bond: [B] 3+ [F] 1 S – O • END = |2.5 – 3.5| = 1.0

• polar covalent bond: S - O

P - H • END = |2.1 – 2.1| = 0.0

• covalent bond: P – H Si - Cl • END = |1.8 – 3.0| = 1.2

• polar covalent bond: Si - Cl Cu - Br • END = |1.9 – 2.8| = 0.9

• polar covalent bond: Cu - Br

N – I • END = |3.0 – 2.5| = 0.5

• polar covalent bond: N - I Br – Cl • END = |2.8 – 3.0| = 0.2

• covalent bond: Br – Cl O – H • END = |3.5 – 2.1| = 1.4

• polar covalent bond: O – H

C – Cl • END = |2.5 – 3.0| = 0.5

• polar covalent bond: C – Cl C – O • END = |2.5 – 3.5| = 1.0

• polar covalent bond: C – O

Covalently-Bonded Structures

   we now have to consider molecules made of several atoms.

most of the following discussion will concern itself with molecules made with covalent or polar-covalent bonds.

ionic bonds (and others) will return later in the unit.

Lewis Structures

  Lewis structures allow us to predict how atoms will come together to make molecules.

Lewis structures are representations of molecules showing all electrons, bonding and nonbonding.

Writing Lewis Structures

PCl 3 5 + 3(7) = 26 1.

  Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.

If it is an anion, add one electron for each negative charge.

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

Writing Lewis Structures

Keep track of the electrons: 2.

The central atom is the

least

electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds.

26  6 = 20

Writing Lewis Structures

3.

Fill the octets of the outer atoms.

Keep track of the electrons: 26  6 = 20  18 = 2

Writing Lewis Structures

4.

Fill the octet of the central atom.

Keep track of the electrons: 26  6 = 20  18 = 2  2 = 0

Writing Lewis Structures

5.

If you run out of electrons before the central atom has an octet… …form multiple bonds until it does.

Exceptions to Octet Rule

  Electron Promotion   group 2 central atom will have 4 electrons group 13 central atom will have 6 electrons Valence Level Expansion     group 15 central atom will have 10 electrons group 16 central atom will have 12 electrons group 17 central atom will have 14 electrons group 18 central atom will have 16 electrons

Writing Lewis Structures

 Write Lewis Structures for the following molecules:

F 2 PCl 3 SeH 2 MgH 2 SiF 4 C 3 H 8 C 3 H 6 C 3 H 4

Coordinate Covalent bonds

 is defined as a covalent bond where one atom provides

both

of the electrons:

Polyatomic Ions

 we can use coordinate covalent bond theory to explain most ions:

Resonance

This is the Lewis structure we would draw for ozone, O 3 .

+

Resonance

 But this is at odds with the true, observed structure of ozone, in which…   …both O—O bonds are the same length.

…both outer oxygens have a charge of  1/2.

Resonance

  One Lewis structure cannot accurately depict a molecule such as ozone.

We use multiple structures, resonance structures, to describe the molecule.

Resonance

Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.

Resonance

  In truth, the electrons that form the second C —O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.

They are not localized , but rather are delocalized .

Resonance

  The organic compound benzene, C 6 H 6 , has two resonance structures.

It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.

Molecular Shapes

  The shape of a molecule plays an important role in its reactivity.

By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

What Determines the Shape of a Molecule?

  Simply put, electron pairs, whether they be bonding or nonbonding, repel each other.

By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.

Valence Shell Electron Pair Repulsion Theory (VSEPR)

“The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

Electron-Domain Geometries

  All one must do is count the number of electron domains in the Lewis structure.

The geometry will be that which corresponds to that number of electron domains.

Molecular Geometries

  The electron-domain geometry is often

not

the shape of the molecule, however.

The molecular geometry is that defined by the positions of

only

the atoms in the molecules, not the nonbonding pairs.

Molecular Geometries

Within each electron domain, then, there might be more than one molecular geometry.

Group 2 Geometries

Central Atom

Magnesium

Bonding Electrons

2

Lone Pair Electrons

0

Bond Type Shape Example

all single linear one double linear MgI MgO 2   In this domain, there is only one molecular geometry: linear.

NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

Group 13 Geometries

Central Atom

Boron

Bonding Electrons

3

Lone Pair Electrons

0

Bond Type

all single one double

Shape

trigonal planar linear one triple linear  because there are no lone pair electrons the molecular is planar (flat).

Example

BI 3 BIO BN

Group 14 Geometries

Central Atom

Carbon

Bonding Electrons

4

Lone Pair Electrons

0

Bond Type

all single one double one triple, or two double

Shape Example

tetrahedral CH 4 trigonal planar COH 2 linear HCN CO 2

Summary for Groups 2, 13 & 14

 because there are no lone pair electrons:     central atom bonded to 1 atom:

linear

central atom bonded to 2 atoms:

linear

central atom bonded to 3 atoms:

trigonal planar

central atom bonded to 4 atoms:

tetrahedral

Group 15 Geometries

Central Atom

Nitrogen

Bonding Electrons

3 5

Lone Pair Electrons

1 0

Bond Type Shape

all single trigonal pyramidal one double angular one triple linear all single trigonal bipyramidal

Example

NH 3 NOH N 2 NCl 5

Group 16 Geometries

Central Atom

Oxygen

Bonding Electrons

2 6

Lone Pair Electrons

2 0

Bond Type

all single angular one double

Shape

linear

Example

H 2 O O 2 all single octahedral OF 6

Group 17 & 18 Geometries

  group 17  normally makes 1 chemical bond (linear)  with valence level expansion can make 7 bonds (ClF 7 ).

group 18   normally makes no bonds with valence level expansion can make 8 bonds (ArF 8 ).

Larger Molecules

  In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

This molecule is tetrahedral about the first carbon, planar trigonal about the second and angular about the single-bonded oxygen.

Larger Molecules

This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

Using VSEPR Theory

 Determine the shape of each molecule, or atom within the molecule:

F 2 PCl 3 SeH 2 MgH 2 SiF 4 C 3 H 8 C 3 H 6 C 3 H 4

Polarity

  previously we discussed bond dipoles.

But just because a molecule possesses polar bonds does not mean the molecule

as a whole

will be polar.

Polarity

  By adding the individual bond dipoles, one can determine the overall

dipole moment

for the molecule. In other words we can see if the dipoles reinforce each other, or cancel out.

Polarity

  

Assignment

Perform the following activities for the molecules at the bottom of the slide: 1.

2.

3.

4.

Draw a lewis structure Draw a structural diagram and indicate shape.

Determine the electronegativity difference for each bond type and determine whether the bond is covalent, polar covalent or ionic Determine if they will be polar, or non-polar.

F 2 SiF 4 PCl 3 C 3 H 8 SeH 2 C 3 H 6 MgH 2 C 3 H 4

Complete “Fun With Balls & Sticks”

Properties of Molecules

Intermolecular Forces

Properties of Molecules

   are determined by how the molecules interact with each other. How they interact is determined by the

forces of attraction

between molecules. These are called

intermolecular forces

; the forces which act

between

molecules, to draw them together, forming the various phases of matter.

How do we measure force ?

    the best way is by

temperature

.

temperature is a measure of kinetic energy; the lowest temperature represents zero kinetic energy.

for a substance to melt or boil the kinetic energy must overcome the intermolecular force.

the higher the melting temperature or boiling temperature, the greater the intermolecular force.

Van der Waals Forces

  occur between covalently bonded molecules.

three kinds:    London disersion dipole-dipole attraction hydrogen bonding

London Dispersion Forces

   are the dominant forces between

covalently bonded, non-polar molecules

based on the formation of

instantaneous dipoles

.

the more electrons in a molecule, the stronger the force.

Relationship of Boiling Point to Number of Electrons and Molar Mass CH C C C C 2 3 4 8 4 H H H H 6 8 10 18

# e Melting Point Boiling Point °C °C

10 18 26 34 66 - 182 - 183 - 190 - 138 - 57 -161 - 88 - 44 0.5

+125

London dispersion forces

 are influenced by

shape

:  Normal Pentane (C 5 H 12 )    m.p. -130C, b.p. 36 °C Neopentane (C 5 H 12 )   m.p. -20 °C, b.p. 9 °C

Dipole-Dipole attraction

   is a force which acts between

polar molecules

(ex. H 2 S). results from the attraction of the opposite poles of the permanent molecular dipoles. These substances generally have higher melting and boiling points than non-polar molecules with similar molecular weights (or numbers of electrons).

Ion-Dipole Interactions

  A fourth type of force, ion-dipole interactions are an important force in solutions of ions.

The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

Dipole-Dipole Interactions

The more polar the molecule, the higher is its boiling point.

How Do We Explain This?

  The nonpolar series (SnH 4 to CH 4 ) follow the expected trend.

The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly.

Hydrogen Bonding

  a specialized form of dipole-dipole attraction. It occurs as when O, N, and F are bonded to H, owing to the large electronegativity difference:    O - H N - H F - H 3.5 - 2.1 = 1.4

3.1 - 2.1 = 1.0

4.1 - 2.1 = 2.0

   This is a stronger force than standard dipole-dipole attraction. Molecules with hydrogen bonding will have boiling points and melting points quite a bit higher than molecules that have only dipole-dipole or London dispersion forces. Hydrogen bonding is responsible for many of the unusual properties of water.

Hydrogen bonding

  is responsible for folding, final structure and function of proteins holds the DNA strands together, but allows them to be unzipped for copying and gene expression.

Relationship Between Polarity, Force and Boiling Point #

e

- B.P. ( °C) Polarity Force C 2 H 6 18 - 161 H 2 S 18 - 60 H 2 O 10 + 100 Non-polar

London

Polar Very polar

Dispersion Dipole - Dipole Hydrogen Bonds

       

Ionic Bonding

generally occurs in compounds of metals and non-metals (salts). It is the result of the attraction of oppositely charged ions. The structures formed are very orderly and are given the name crystal lattice. Ionic solids are called crystals. No sharing of electrons occurs between the ions in the crystal lattice. As a result, ionic solids are brittle. Ionic solids conduct electricity only in the molten state, and not very well. Ionic solids are characterized by very high melting and boiling points.

     

Metallic Bonding

is the bonding which occurs between metals in the Periodic Table. It is characterized by close packing of the atoms, with the electrons delocalized; that is, they are free to jump from atom to atom, filling unoccupied orbitals. This free sharing of electrons allows metals to conduct electricity freely (copper conducts electricity 100 000 times better than molten NaCl). The free electrons also act as a lubricant, allowing metal atoms to slide over one another without affecting the integrity of the material. Thus metals are malleable and ductile. This bond is strong, giving most metals high melting and boiling points.

This bond is also variable.

     

Network Covalent Bonding

This is the traditional covalent bond, expanded to 2 or 3 dimensions in a network which is theoretically infinite (much like an ionic crystal lattice). Network solids include diamonds, graphite, quartz, and most rocks. Because the covalent bond is stronger than any other bond, the network solid is very hard (diamonds are the hardest substance known). Because electrons are held tightly in their bonds, network solids are brittle, and they do not conduct electricity. Because the orientation of the atoms is very specific to the bonding orientation (tetrahedral, planar trigonal), network solids form distinct crystals. Because of the strength of the bonds, network solids have very high melting and boiling points

Summarizing Intermolecular Forces

Intermolecular Forces Affect Many Physical Properties

The strength of the attractions between particles can greatly affect the properties of a substance or solution.

Viscosity

• Resistance of a liquid to flow is called viscosity .

• It is related to the ease with which molecules can move past each other.

• Viscosity increases with stronger intermolecular forces and decreases with higher temperature.

Surface Tension

Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.

Vapor Pressure

• At any temperature, some molecules in a liquid have enough energy to escape.

• As the temperature rises, the fraction of molecules that have enough energy to escape increases.

Vapor Pressure

As more molecules escape the liquid, the pressure they exert increases.

Vapor Pressure

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense

at the same rate .

Vapor Pressure

• The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure.

• The normal boiling point is the temperature at which its vapor pressure is 760 torr.

Intermolecular Forces in Summary

Strength of Attraction

Molecular

(London dispersion)

Molecular

(dipole-dipole)

Molecular

(hydrogen bonding)

Ionic Metallic Network Covalent

1 to 50 5 15 50 20 to 80 100

Melting and Boiling Point

Molecular

(London dispersion)

Molecular

(dipole-dipole) very low (nitrogen boils at - 196  C) VARIABLE low (H 2 S boils at -61  C)

Molecular

(hydrogen bonding) medium (H 2 O boils at +100  C)

Ionic Metallic Network Covalent

high (NaCl boils at +1413  C) variable (Hg @ +357  C, W @ +5660  C) very high (SiO 2 boils at 2600  C)

Properties of Solids

Molecular

(London dispersion) soft, waxy

Molecular

(dipole-dipole) more rigid

Molecular

(hydrogen bonding) crystalline, brittle

Ionic Metallic Network Covalent

long-range crystalline, hard, brittle short-range crystalline, ductile, malleable long-range crystalline, hard, brittle

Conductance of Heat and Electricity

Molecular

(London dispersion) non-conductor

Molecular

(dipole-dipole) non-conductor

Molecular

(hydrogen bonding) non-conductor

Ionic Metallic Network Covalent

conductor in liquid or dissolved phase conductor in solid or liquid phase non-conductor in 3d form, some conductance in 2d

Molecular

Solubility in H

2

O

Not soluble (London dispersion)

Molecular

(dipole-dipole) Soluble

Molecular

(hydrogen bonding) Soluble

Ionic

Soluble

Metallic Network Covalent

Not soluble Not soluble