Chemical Bonding. © 2014 Pearson Education, Inc. Types of Bonds We can classify bonds based on the...

Chemical Bonding

© 2014 Pearson Education, Inc.

Types of Bonds

• We can classify bonds based on the kinds of atoms that are bonded together.

Copyright © Houghton Mifflin Company. All rights reserved. 5 | 3

The Relationships Among the Three Extreme Bonding Types:

Covalent, Ionic, and Metallic


Metallic Bonding Dominates in Bonds Among Elements on the Left Side of the Periodic Table. Covalent Bonding to the Upper Right. Elements from Opposite Sides bond Ionic fashion.(Why?)


Elements on the Left-hand Side of the Periodic Table have a Few Loosely Held Valence Electrons


Electronegativity Scale


Electronegativity Difference and Bond Type

• If the difference in electronegativity between bonded atoms is 0, the bond is pure covalent.– Equal sharing

• If the difference in electronegativity between bonded atoms is roughly 0.5 to 1.9, the bond is polar covalent.

• If difference in electronegativity between bonded atoms is larger than or equal to 2.0, the bond is mostly or strongly ionic.


Percent Ionic Character in a Single Chemical Bond

The Electronegativity Different between two elements determines the percent ionic character.Examples:

Substance Electronegativity Difference

% Ionic Character

NaCl 3.10 – 0.93 = 2.17 70%

N=O 3.44 – 3.04 = 0.40 4%


(a) The Electrostatic Charge Density Surfaces for NaCl, HCl, and Cl2 Show the Differences Between Ionic (NaCl), Mainly Covalent (HCl) and Purely Covalent (Cl2 ) Bonds.

(b) The Electron Density Surface Presents Another Picture for the Ionic Compound NaCl Compared with Covalent HCl and l2.

Electrostatic Surfaces Demonstrate Ionic Character.


Interaction Between Two Atoms Showing the Polarization Due to the Coulombic Attraction of Each Electron Cloud for the Nucleus of the Approaching Atom

Note: This is the conventional explanation and is not entirely true. In fact, it is the superposition of electrons around both nuclei simultaneously that is responsible..a purely quantum mechanical effect.

How do Covalent Bonds Form?


Like Two Water Waves, Electron Waves in Two Different Atoms Interact as the Atoms Come Together.

Also, Waves can combine Constructively or Destructively.

How do Covalent Bonds Form?


(Left column) Two Hydrogen Atoms Approaching with 1s Orbitals in Phase Result in an Enhanced Amplitude in the Internuclear Space. This results in a “Bonding” Molecular Orbital.

(Right column) Out-of-phase Orbitals in the Two Approaching Hydrogen Atoms Cancel Each Out in the Internuclear Space, Resulting in Diminished Amplitude, or a node between the Atoms. This results in an “Anti-Bonding” Molecular Orbital.


The study of bonding is carried out using Molecular Orbital Theory.

The Molecular Orbital Energy-level Diagram for Formation of a Diatomic Molecule from Two Atoms, Each with a Single s Orbital, Shows the Energy of the Separate Atomic Orbitals on Each Side of the Diagram

Molecular Orbital Theory

The bonding orbital resulting are called and * for bonding and anti-bonding, respectively.


1.) Electrons are Indicated in the Energy Level Diagram as Arrows.2.) Electrons are combined from individual atoms into the newly formed molecular orbitals. 3.) As in atoms, the electrons fill from the lowest energy up, spin paired and the Pauli-Exclusion Principle applies.



The strength of the bond is called the “bond order” and is given as:

BO = ½ (# electrons in bonding orbitals - # electrons in anti-bonding orbitals)


So for hydrogen molecules, the bond order is:

½ (2 – 0) = 1 or a single bond.


The He2 Molecular-orbital Energy-level Diagram Features a Filled Bonding and a Filled Antibonding Molecular Orbital Resulting in an Unstable Dimer

So for helium, the bond order is:

½ (2 – 2) = 0. So we see why we don’t find diatomic helium.


Formation of Molecular Orbitals from Two pz Atomic Orbitals

Just as “s” atomic orbitals combine to form bonding and anti-bonding orbitals, so do the “p” atomic orbitals. Here we have “p” orbitals aligned along the bond. These are called p and p

* for bonding and anti-bonding orbitals, respectively.


In-phase Atomic px and py Orbitals are perpendicular to the bond. These can still overlap to produce a bonding and anti-bonding orbitals. These types are called “” orbitals and

given the designation and * for bonding and anti-bonding orbitals.


The Molecular Orbital Energy-level Diagram showing the Relative Energies of the Orbitals resulting from “p”

Orbital Overlaps.


The Molecular Orbital Energy-level Diagram showing the Relative Energies of all of the and Orbitals.

The Molecular Orbital Energy-level Diagram is filled for the O2 Molecule.Notice that only Valence Electrons are included.

All the core electrons do not contribute significantly to bonding.


Notice that in O2, the last two electrons are put in separate orbitals with parallel spins. Spinning electrons generate magnetic fields and, as such, O2 has a magnetism and is said to be “paramagnetic”.The ability of Molecular Orbital Theory to predict this was one of its’ successes.


The unpaired electrons give O2 a bluish color and allows it to be suspended in a magnet.


The Responses of Different Materials to a Magnetic Field Reveal

Pairing of Electrons in the Bonds


The Bond Length of Second-period p-block Homonuclear Diatomic Molecules Decreases as the

Bond Order Increases from B Through N, then Increases Again as the Antibonding Orbitals Fill


The Molecular Orbital Energy-level Diagram for Second-period Diatomic Molecules involving Li2, Be2, B2 and C2 have the bonding p bonding orbitals at

a lower energy than the p bonding orbitals


The Molecular Orbital Energy-level Diagram for N=O Illustrates the Effect of Differing Electronegativities on Bonding

Greater nuclear charge draws electrons closer and results in lower orbital energies. This can have a tremendous effect on which orbitals mix.

Metallic Bonding and

Modern Materials


Bonding in Beryllium

The bond order in Beryllium shows that a stable bond will not form. Yet Be metal exists as a stable substance! Why is this?


Band TheoryWhen more than two atoms produce molecular orbitals, the atomic waves continue to

combine.

As the number of atoms in a chain increases, the energy gap between molecular orbitals (MOs) essentially disappears, and continuous bands of energy states result.

In Li, the result in a set of filled and unfilled orbitals. It allows electrons to flow resulting in Li being conductive. This is the MO model of the “electron sea”.


Processing Flow View from Atoms to Solid for Be.

For Avogadro’s number of Be atoms (NA), we see the same trend.HOWEVER: All the molecular orbitals are filled! This implies that a.) The electrons are in bonding and antibonding orbitals and a zero bond order and b.) Even if we had bonds, the electrons have nowhere to move and Be should therefore be non-conducting.

But Be does exist as a metal and is actually twice as conductive as Li. So again we see an apparent contradiction from observation. What is missing?


Processing Flow View from Atoms to Solid for Be.

In fact, we have forgotten that we also have “p” orbitals that can become involved in bonding.

“s” and “p” orbitals can “mix” to produce hybrids and the result leads to a greater bonding potential.


Processing Flow from a Mole of Atoms to the Solid for Be, Including the Valence 2p Orbitals

Now we see a large set of bonding orbitals forming a band called a “valence” band and anti-bonding set called the “conduction” band. The reasons for these names will become apparent soon.All the electrons are now seen to be in bonding orbitals. Thus Be metal is stable.


Elements and the Band GapExamination of the bands shows that there is an apparent energy gap between the two bands. This is, in fact quite true, but the energy gap differs tremendously for different elements. In general:•The closer the atoms are together, the greater the band gap.•The greater the difference in electronegativity, the greater the band gap.

For metals in general, the band gap is extremely small allowing electrons to bridge the gap and become conductive.For non-metals the band gap increases significantly and electrons are unable to access the upper band. Thus non-metals are generally nonconductive.Of particular interest are those elements that have ½ filled atomic orbitals. A good example is carbon.


Processing Flow from Atoms to Covalent Solid for Diamond

Note two features of this processing flow1.) The valence band is completely filled. Thus the electrons have nowhere to move. Thus carbon is non-conducting.2.) The band gap is now larger than Be. For carbon it is 5.4 eV.


The Band Gap and Light - SemiconductorsIn order to make these materials conduct, electrons from the valence band must be excited to the conduction band. This can be done with direct voltage or by light among other things. Materials can absorb light energy if the band gap is in the range of the light energy. This gives many objects their color.

For light, E = hc/. = 1240.8 eV-nm/ where h is Planck’s constant and c is the speed of light. is the wavelength and thus the color of the light. Red light has a wavelength of about 650 nm, so a material will absorb red light if the band gap is:

1240.8 eV-nm/650 nm = 1.91 eV. We will see soon that the color of diodes is due to this effect. Diamond requires a wavelength of 230 nm to absorb. That is in the far ultraviolet and so diamond is transparent.

List of Semiconductor Materials


Band Gap in Group 14 Diamond-structure Solids

The band gap size trend can be clearly seen. Atoms are farther apart as one moves down the Group and thus the band gap decreases.


• What is the atomic density of Carbon and Germanium? Does the atomic density correlate with the different band gaps?

(Answer: C: 113 atoms/nm3, Ge: 44 atoms/nm3.

Yes! More atoms/nm3 means closer together and thus a high band gap)

• The band gap of Ge is 0.67 eV. What wavelength of light is required to excite an electron across this gap?

hc = 1240.8 eV-nm(Answer: 1852 nm infrared)


Mixed-Valence Semiconductors

• ½ Filled shells can be achieved by mixing elements to create the proper balance.

• For example a 50:50 mixture of Ga and P will work. This is because Ga is one electron short of a ½ filled shell and P is one electron high. Thus mixing the two evens out the electrons to a total of ½ filled.

• The advantage of this is that mixtures of elements can be made that will produce band gaps of different sizes as each element has a different size and the electronegativity difference is varied.

• Let’s examine the GaP system.


Processing Flow from Atoms to Covalent Solid for GaP

The valence band is again filled while the conduction band is empty. However because of the relative sizes of Ga and P and the electronegativity difference, the band gap is only about 2.5 eV which is the energy of green light. Green diodes are made of this semiconductor.


The Variations in the Band Gaps in Binary Group III-V (13-15) Semiconductors


Processing Flow from Atoms to Covalent Solid for ZnSe

Here we see the same effect. Zn is 2 electrons short and Se is 2 electrons beyond a filled shell. The band gap is 2.58 eV.


The Variations in the Band gaps in binary Group II-VI (12-16) Semiconductors


Fine Tuning Band GapsConsider GaP and GaAsBoth P and As appear in Group 16 and, as we’ve’ seen, produce semiconductors having different band gaps and color.Band Gaps: GaAs = 1.35 eV (919 nm – infrared), GaP = 2.24 eV (560 nm – Yellow)What if we were to create a semiconductor having both P and As in with the Ga? Depending on the relative amounts of P and As, the semiconductor will vary smoothly between the extremes.Thus a semiconductor designated GaP0.40As0.60 might have a color:

∆𝑬= 𝟏.𝟑𝟓 𝒆𝑽+ ሺ𝟐.𝟐𝟒 𝒆𝑽− 𝟏.𝟑𝟓 𝒆𝑽ሻ∙(𝟎.𝟒𝟎) = 𝟏.𝟕𝟎𝟔 𝒆𝑽

𝝀= 𝒉𝒄∆𝑬= 𝟏𝟐𝟒𝟏.𝟒 𝒆𝑽∙𝒏𝒎𝟏.𝟕𝟎𝟔 𝒆𝑽 = 𝟕𝟐𝟖 𝒏𝒎− 𝑹𝒆𝒅

Note: This is very approximate as the relationship is not necessarily linear.


Color Variations


The search for the blue semiconductor.Blue diode lasers are potentially quite valuable. The shorter wavelength means the ability to read digital data to much higher precision. The result is storage capacity of DVD’s orders of magnitude high. Nitrogen semiconductors are currently being explored for this purpose.


Mixing unequal combinations of elements can create excess electrons or holes.

(a) Pure Silicon Features a Filled Conduction Band and an Empty Valence Band. (b) Doping with P adds Electrons to the Conduction Band Creating a n-type Semiconductor. (c) Substitution of Al for Si Leaves the Valence Band Incompletely Filled, Creating a p-type Semiconductor

The doped semiconductors now have electrons that are mobile. Thus higher conductivity

Semiconductor Doping and Conductivity


The Range of Resistivity of Materials Varies Widely


Diodes

One of the great uses of semiconductors is the Diode. A diode is formed from an n-type and p-type semiconductor producing an p-n junction.

•On the n-type side there are mobile electrons in the conduction band.

•On the p-type side the mobile electrons are in the valence band.

•These two bands have different energies, the difference of which is the band gap energy.

•Conduction band electrons of the n-type semiconductor are pushed, via a voltage source, into the p-n- junction where they lose energy falling into the valence band of the p-type semiconductor.

•The loss of energy gives off light, the color of which depends on the band gap.


Forward Bias p-n JunctionHere electrons flow from left to right, falling into the valence band. Note the attachment of the battery. Electrons always flow towards the cathode or “+” lead.


Reverse Bias p-n JunctionIf an attempt to push electrons in the opposite direction, there is a large energy barrier. Generally the energy used to drive the electrons is dispersed as heat and the diode shorts out. Thus electron flow can only occur in one direction.


The Blue Diode and Information Storage

Take any CD and hold it at an angle to a light and observe the rainbow of colors. The rainbow results from light bouncing off the closely spaced ridges on the CD.

Minute pits in these ridges store the information. Reducing the spacing between the ridges and making the pits more dense within the ridges, increases the amount of information that can be stored.

Current devices are read by lasers – infrared lasers (780 nm) for CDs and red lasers (650 nm) for DVDs and bar-code readers that you see in many stores.

The source of light in these lasers is based on the semiconductor diode.


Structure of a CD


Optical Storage Devices

CDs are read by infrared lasers with a wavelength of 780 nm. Storage capacity: 650 MB (megabytes) enough to hold two sides of an LP with a little space left over.

DVDs are read by red lasers with a wavelength of 650 nm. Storage capacity 4,700 MB, enough to hold a short movie.

Future device will be read by a blue laser of wavelength 400-450 nm. Capacity 16,000 MB, enough to hold the entire “Star Wars” collection with sound track and special features!

Pit

Space Between Ridges

Ridge


Optical Storage Capacity of Blue Diode Lasers.

A shorter wavelength diode laser can read more pits/cm2.

If a laser can successfully read pits that are 4 times the wavelength along a track and 10 times the wavelength between tracks, how much more information can be store with a blue laser of 6 eV band gap or 207 nm as compared to a DVD of wavelength 650 nm which has a storage capacity of 4.7 Gigabytes?

The pits in a DVD are (4 x 650 nm) = 2600 nm/pit.

Inverting gives 1 pit/2600 nm ((107 nm/cm) = 3850 pits/cm along a track.

By similar analysis there are 1540 pits/cm across tracks.

The total is (3850 x 1540) = 5.9 x 106 pits/cm2.

In a blue laser optical storage device, the number of pits/cm2 becomes 5.8 x 107 pits/cm2. Thus we have:

(5.8 x 107/5.9 x 106) x 4.7 Gb = 46.5 Gb storage capacity!

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