Bonding in Solids- 11.8

By: Jin Lee, Paul Gregotski, Kayla Seider, and Hannah Cherry

Molecular Solids• Molecular solids consist of atoms or molecules held together by intermolecular force

– Dipole-dipole forces– London dispersion forces– Hydrogen bonds

• Molecular solids are often soft because of these weak forces– Most solid at low temperatures and low melting points

o ex) Ar, H2O, CO2• These properties also depend on their ability to be packed efficiently within a 3D space

– Benzene (C6H6) is highly symmetrical and planar – Benzene has a higher melting point than toluene – Toluene is a compound where an H atom of Benzene is replaced with a CH3 group

o The lower symmetry of toluene prevents it from packing efficientlyo The intermolecular forces that need to be close are weakened

• However the boiling point of toluene is greater than benzene– Shows that toluene has stronger forces in liquid state

• Phenol, another benzene like molecule, with an OH group– Has higher melting and boiling point than both toluene and benzene– OH group in phenol creates hydrogen bonds

Covalent-Network Solids• Covalent-Network Solids consist of atoms held together in large networks or chains by covalent bonds

– These bonds are much stronger than intermolecular forces– These solids are harder and have higher melting points than molecular solids

• Diamond and Graphite, two allotropes of carbon, are covalent-network solids– allotrope: one of many forms in which a chemical element occurs, each differing in physical properties, e.g.

diamonds and coal as forms of carbon– Other examples are SiO2 (quartz) and SiC (silicon carbide) and BN (boron nitride)

• Diamond vs. Graphite– Diamond

o Each carbon atom is bonded to four other carbon atomso Creates a strong interconnected 3D array of carbon-carbon single bonds

This leads to diamond‘s hardness Also leads to a 3550 degrees Celsius melting point

– Graphite• Carbon atoms are arranged in layers of hexagonal rings• Each carbon is bonded to three others in the layer

– The distance between adjacent carbon bonds is 1.42 Angstroms» Relatively close to C-C bonds in benzene of 1.395 Angstroms» Bonding on benzene and graphite both contain delocalized pi bonds extending over the layers

– Electrons also move freely from layer to layer » This makes it a good conductor of electricity

– Layers are 3.41 Angstroms apart and held together by weak dispersion forces» This causes the layers to slide by each other

• It is used as a lubricant• Also used as the “lead” in pencils

Ionic Solids• Ionic Solids consist of ions held together by ionic bonds• The strength of a bond depends on the charges of the ions

– NaCl with ions of 1+ and 1- has a melting point of 801 Celsius– MgO with ions of 2+ and 2- has a melting point of 2852 Celsius– Structures of ionic solids are categorized into a few basic types

• NaCl is a representative of one type– Other compounds include: LiF, KCl, AgCl, and CaO

• Other common types of structures are such as ZnS and CaF2– These structures depend on the relative size of the ion and charge

• NaCl and CsCl– NaCl, the Na+ is surrounded by 6 Cl atoms

» Giving it a corrdination number of 6– CsCl, the Cs+ is surrounded by 8 Cl atoms with a coordination number of 8

» This occurs because Cs is relatively larger than Na• ZnS structure

– This structure adopts a face centered cubic arrangement– Zn2+ are surrounded by 4 S2-

» CuCl also adopts this structure too• CaF2 structure

– The Ca2+ ions are in a face centered cubic arrangement– There are twice as many F atoms

» BaCl2 and PbF2 also adopt this structure.CaF2, twice as many F atoms as Ca atoms

.ZnS, Zn2+ is surrounded by 4 S2- in this structure

.CsCl structure, Cs+ has a coord. # of 8

.NaCl structure, Na+ has coord. # of 6

Metallic SolidsMetallic solids consist entirely of metal atoms

• Usually consist of hexagonal, close-packed, cubic close-packed or body centered cubic structures

• Each atom usually has 8 or 12 adjacent atomsMetallic bonds are created due to electron that are delocalized

throughout the entire solid• This image can be created by imagining positive ions immersed in a sea of

delocalized electrons Metals also vary greatly within their physical properties

• Mostly depends on number of electrons there are to move around• Na, only has 1 valence electron, thus is melts at 97.5 Celsius• Chromium has 6 valence e- beyond noble gas core, melting at 1890 Celsius• These delocalized electrons give reason to high conductibility and good

conductors of heat metals are

.Metallic Solids, the kernels represent + ions while the blue fog is delocalized e-

More StructuresPerovskite Structure (ionic crystal)

• The lattice is essentially cubic primitive, but may be distorted to some extent and then becomes orthorhombic or worse. It is also known as the BaTiO3 or CaTiO3 lattice and has three different atoms in the base

Spinel Structure (ionic crystal)• The spinel structure (sometimes called garnet structure) is

named after the mineral spinel (MgAl2O4); the general composition is AB2O4. It is essentially cubic, with the O - ions forming a face centered cubic lattice. The cations (usually metals) occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites and there are 32 O-ions in the unit cell

. Pervskite Strucuter, 3 elements with 3 different bases

More Structures

In-depth analysis of NaCl and CsCl Why is sodium chloride 6:6-co-ordinated?

• The more attraction there is between the positive and negative ions, the more energy is released.

• The more energy that is released, the more energetically stable the structure becomes• That represents the maximum number of chloride ions that you can fit around a

central sodium ion before the chloride ions start touching each other. • If they start touching, you introduce repulsions into the crystal which makes it less

stable Why is CsCl an 8:8 while NaCl is 6:6?

• Which structure is more likely a 1:1?o Depends greatly on the radius of the atomso Cs is 93% of the size of Cl while Na is 52%

The different sizes of these ions can accommodate for different amount of Cl- ions• Too many results in repulsions because Cl would be too close

Unit CellCrystalline solids have a regular repeating pattern. Amorphous solids, or non-crystalline solids, are randomly arranged. Examples are quartz and glass. Both are silicon dioxide, but the quartz has a hexagonal crystal structure. This makes it have a sidedness and a grain. When you strike a piece of quartz, it will break along a plane. In contrast, glass is quartz that has been heated until it melts, scrambling the crystal structure, then cooled quickly before the atoms can line up in a regular array. Glass doesn’t have a grain. It can be cut in any direction even a circle. When it breaks, it breaks in random patterns rather than along lines of cleavage.

The Unit Cell

Step 1:Step 2:Step 3:Step 4:Step 5:

More on Metallic Bonds The cations within a metallic solid are known as Kernels.

• The larger the magnitudes of the positive charge on the metallic nuclei, the greater the strength of the metallic bond.

• The greater the number of valence electrons contributed to the electron sea, the greater the strength of the metallic bond.

Metallic bonds are omnidirectional. They do not have any geometric requirements which need to be fulfilled• If a metal is subjected to a force, the kernels can slide around

on the layer of electrons• As the kernels move to new positions, the bonds will not

break, because of their omnidirectional nature

.Electrons in a metallic solid are loose and omnidirectional

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Metallic Bonding

Summarize

What type of cubic crystal is represented by the following diagram?a.)Normalb.)Cubic

c.)Face-Centeredd.)Body-Centered

e.)Simple

Q&A:

c.) face-centered

Which type of crystalline solid is SO2 most likely to form?a.)Ionic

b.)molecularc.)atomic

d.)metallice.)covalent

b.)molecular

Which of the following substances should have the highest normal boiling point?

a.)CH3Clb.)CH3Brc.)CH3Id.)CH4

e.)CH3OH

e.)CH3OH

Which substances crystallizes as molecular solid?a.)Arb.)Ni

c.)NaCld.)phosphorous

e.)NO

e.)NO

Bibliography• Brown, Theodore L., H. Eugene LeMay, Bruce E. Bursten, and Julia R. Burdge.

Cehmistry. Ninth ed. Upper Saddle River: Prentice Hall. Print. The Central Science.

• "Ionic Crystals." Technische Fakultät | Homepage. Web. 26 Nov. 2011. <http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html>.

• "Ionic Structures." Chemguide: Helping You to Understand Chemistry - Main Menu. Web. 26 Nov. 2011. <http://www.chemguide.co.uk/atoms/structures/ionicstruct.html>.

• "Metallic Bond." Redirect. Web. 26 Nov. 2011. <http://mysite.verizon.net/kdrews47/solids/metallics.html>.

• "Solids and Symmetry." Angel C. De Dios, Department of Chemistry, Georgetown University. Web. 28 Nov. 2011. <http://bouman.chem.georgetown.edu/S02/lect30/lect30.htm>.