Bonding in Solids 11.8
v Physical properties within solids depend on arrangement or particles and their attractive forces within
v Molecular Solids
Ø Molecular solids consist of atoms or molecules held together by ______forces
§ Dipole-dipole forces
§ London dispersion forces
§ Hydrogen bonds
Ø Molecular solids are often soft because of these weak forces
§ Most solid at low temperatures and low melting points
· ex) Ar, H2O, CO2
Ø These properties also depend on their______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
· The lower ______of toluene prevents it from packing efficiently
· The______forces that need to be close are weakened
§ However the boiling point of toluene is greater than benzene
· Shows that toluene has ______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 ______bonds
v Covalent-Network Solids
Ø Covalent-Network Solids consist of atoms held together in large ______or ______by covalent bonds
§ These bonds are much ______than intermolecular forces
§ These solids are harder and have higher melting points than______solids
Ø 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)
Ø Diamond vs. Graphite
§ Diamond
· Each carbon atom is bonded to four other carbon atoms
· Creates a strong ______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 ______rings
· Each carbon is bonded to ______others in the layer
¨ The distance between adjacent carbon bonds is 1.42 Angstroms
Ø Relatively close to C-C bonds in benzene of 1.395 Angstroms
Ø Bonding on benzene and graphite both contain ______pi bonds extending over the layers
¨ Electrons also move freely from layer to layer
Ø This makes it a good conductor of electricity
¨ Layers are 3.41 Angstroms apart and held together by weak ______forces
Ø This causes the layers to slide by each other
§ It is used as a lubricant
§ Also used as the “lead” in pencils
v Ionic Solids
Ø Ionic Solids consist of ions held together by______bonds
Ø 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 ______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
Ø These structures depend on the relative______of the ion and charges
§ NaCl and CsCl
· NaCl, the Na+ is surrounded by 6 Cl atoms
¨ giving it a______number of 6
· CsCl, the Cs+ is surrounded by 8 Cl atoms with a coord. # of 8
¨ This occurs because Cs is ______larger than Na
§ ZnS structure
· this structure adopts a ______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 ______atoms
¨ BaCl2 and PbF2 also adopt this structure
v Metallic Solids
Ø Metallic solids consist entirely of metal atoms
§ Usually consist of hexagonal, close-packed, cubic close-packed or ______centered cubic structures
§ Each atom usually has 8 or 12 adjacent atoms
Ø Metallic bonds are created due to electron that are delocalized ______the entire solid
§ This image can be created by imagining positive ions______in a sea of delocalized electrons
Ø Metals also vary greatly within their physical properties
§ Mostly depends on number of ______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 ______and good conductors of heat metals are
v More Structures:
Ø Perovskite Structure (ionic crystal)
§ The lattice is essentially ______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 ______atoms in the base
Ø 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 ______, with the O - ions forming a face centered cubic lattice. The cations (usually metals) occupy 1/8 of the ______sites and 1/2 of the octahedral sites and there are 32 O-ions in the unit cell
v In-depth analysis of NaCl and CsCl
Ø Why is sodium chloride 6:6-coordinated?
§ 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 ______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 ______into the crystal which makes it ___ stable
Ø Why is CsCl an 8:8 while NaCl is 6:6?
§ Which structure is more likely a 1:1?
· Depends greatly on the radius of the atoms
· Cs is 93% of the size of Cl while Na is 52%
§ The different sizes of these ions can accommodate for different ______of Cl- ions
· Too many results in repulsions because Cl would be too close
v More on Metallic Bonds
Ø The cations within a metallic solid are known as Kernels.
§ The larger the ______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 ______of the metallic bond.
Ø Metallic bonds are ______. They do not have any geometric requirements which need to be fulfilled
§ If a metal is subjected to a force, the kernels can ______around on the layer of electrons
§ As the kernels move to new positions, the bonds will not break, because of their omnidirectional nature