Bonding in Solids Sovay, Jen and Miranda

Overview Physical properties of crystaline solids, such as melting point and hardness depend on the arrangements of particles and on the attractive forces between them

Molecular Solids Molecular solids consist of atoms or molecules held together by intermolecular forces Intermolecular forces Dipole: dipole forces, london dispersion forces, and hydrogen bonds Because forces are weak, Molecular solids are soft

Properties of Molecular Forces Depend on strengths of the forces that exist between molecules (Intermolecular forces that depend on close contact are not as effective, the melting pt is lower)

Covalent-Network Solids -Consist of atoms held together by covalent bonds -stronger than intermolecular forces -these solids have higher melting points and are harder than molecular forces

Ionic Solids -Ions held together by ionic bonds -Strength depends on charges of ions -Structure can be classified as few basic types

Basic Types of Ionic Structures NaCl, HF, KCl, AgCl, and CaO For Na : each cation is surrounded by it’s six neighboring anions because the Na ions have a coordination number of 6 CsCl - Each Cs ion is surrounded by eight CL, Coordination # is 8 for CS Difference in coordination number is accounted for by the larger size of Cs ZnS – Each of the small Zinc ions are tetrahedrally surrounded by four S ions (face centered cubic) CaF2, BaCl2, PbF2 – (face centered cubic) all have twice as many anions as cations

Metallic Solids -Consist entirely of metal atoms -Has several structures - Bonding too strong to be due to London Dispersion Not enough valence e- to be due to ordinary covalent bonds

Metallic Solids (continued) Bonding is due to valence electrons that are delocalized throughout the entire solid Vary greatly in bonding strength Mobility of electrons

Molecular Solids Bonding Energy Dispersion – 1.0 KJ/mole Hydrogen Bond – KJ/mole Ionic – KJ/mole Covalent – KJ/mole orces/Bonding

Bonding in Solids Cheat Sheet Type of Crystal Forces holding units together General Properties Examples Molecular Inter - molecular forces Soft, low melting point, poor conductor of heat and electricity Ar, CO2, I1, H2O, C12H22O11 IonicIonic bonds Hard, brittle, high melting point, poor conductor of heat and electricity NaCl, LiF, MgO, CaCO3 CovalentCovalent bonds Hard, high melting point, poor conductor of heat and electricity C (diamond or graphite) Si, SiO2 MetallicMetallic bonds Soft to hard, low to high melting point, good conductor of heat and electricity All metallic elements (Na, Mg, Ti, Cu, Fe…) cs/Bonding_In_Solids/primary_bonding_summary.pdf

Bond Type Triangle Shows classification of bonding types based on average electronegativity differences chem.purdue. edu/genchem/ topicreview/b p/materials/m aterial1.html

Create your own CsCl Model CsCl is an ionic crystal with a coordination number of six. The red and yellow gummy bears are the Cl- and the Blue Gummy bear is the Cs+

Questions What kinds of attractive forces exist between particles in a) molecular crystals; b) covalent network crystals; c) ionic crystals; d) metallic crystals A) hydrogen bonding, dipole dipole forces, London dispersion forces B) covalent chemical bonds C) Ionic bonds D) Metallic Bonds

Questions Covalent bonding occurs in both molecular and covalent network solids. Why do these two kinds of solids differ so greatly in their hardness and melting points? In molecular solids, relatively weak intermolecular forces bind the molecules in the lattice, so it requires little energy to disrupt these forces. In covalent network solids, covalent bonds join atoms into an extended network. Melting or deforming a covalent network solid requires breaking these bonds, which requires a large amount of Energy

Questions A white substance melts with some decomposition at 730 degrees Celsius. As a solid, it is a nonconductor of electricity, but it dissolves into water to form a conducting solution. Which type of solid might this be? Must be ionic because of relatively high melting point and properties as a conducting solution

Questions Which will have a higher melting point and why? A) B, BF3; B) Na, NaCl; C) TiO2, Ti Cl4 A) B, covalent network lattice like C(s), versus weak dispersion forces in BF3 B) NaCl, ionic vs metallic bonding C) TiO2, higher charge on the O2