Bonding, Intermolecular Forces, and Solids

Bonding, Intermolecular Forces, and Solids SCH4U0

Bonding • From valence bond theory we have learned that: • Bonds are formed when orbitals overlap • Electrons are filled into the resulting molecular orbital • We also know (from grade 11) that not all bonds are the same • Atoms have different electronegativities • The electronegativity shows how strongly an atom can pull the electrons in a bond towards itself • This results in a shift in the electron density of the bond

Molecular Orbital Electron Density • The difference in electronegativity (ΔEN) of the two atoms determines how shifted the electron cloud is Polar Covalent Bond Ionic Bond 0 0.4 1.7 3.3 Pure Covalent Bond ΔEN Mostly ionic Partially ionic 50% 0 12% 100% % ionic character

Intermolecular Forces • Bond character effects the interaction of molecules • Bonds with high ionic character have electrostatic attractions with one another • These bonds generate electric fields called dipoles • Molecules can be classified as polar or non-polar • This is based on whether or not the molecule as a whole has a dipole

Polar Molecules • Water is an example of a polar molecule because it has a total dipole • The dipoles formed from polar bonds add together to make a total dipole moment δ- O Total Dipole Moment H H δ+ δ+

Non-Polar Molecules • Carbon dioxide also has polar bonds • But the dipoles are exactly opposite to one another and cancel each other out δ+ δ- δ- C O O Total Dipole moment = ZERO

Symmetry • The easy way to tell if a molecule is polar or not is via symmetry • Symmetry refers to mirror images • An object is symmetric if two sides of it are mirror images of one another • Carbon dioxide is symmetric if you look at any plane that runs along the bonds • Since it is symmetric, it is non-polar (dipoles cancel out) • Water is symmetric in two planes, but that is it • Since it is NOT very symmetric it is polar (the dipoles won’t cancel out)

Intermolecular Forces • All molecules exert attractions on one another • These are called intermolecular forces • Ionic compounds display ion-ion forces • The attraction of a cation (+) with an anion (-) • Polar molecules exert dipole-dipole forces • The attraction of partial charges δ- δ+

Intermolecular Forces • Polar molecules with highly positive hydrogen atoms have hydrogen bonds • A strong dipole-dipole attraction due to the exposed nucleus of hydrogen

Boiling Points • Hydrogen bonds are stronger than regular dipole forces • This can be seen in the boiling points of certain polar molecule families

London Dispersion Forces • The weakest of the forces are the London dispersion forces • These are the attractive forces between any two molecules, even without dipoles • The electron cloud of a bond is constantly shifting, resulting in minor partial charges • These can attract one another

London Dispersion Forces • There are two main things that affect the strength of London dispersion forces • Area of contact between molecules • Polarizability • Area of contact • The more area, the more attractions can be formed • This is reflected in the difference in BP

Polarizability • Polarizability refers to how easily the electron cloud can be shifted • The more electrons (and shells), the easier it is to shift the electron cloud to become polar • Therefore, iodine is more polarizable than chlorine • This is why iodine is a solid at RT and chlorine is a gas • The London forces are different strengths

Summary

Solubility • Solubility refers to how easily a solute will dissolve into a solvent (and how much will dissolve) • This is determined entirely by intermolecular forces • For a substance to dissolve, there are three things that must occur; • Solute-solute forces must break • Solvent-solvent forces must break • Solute-solvent forces must form

Solubility Solvent Solute Solution

Solubility • A solute will be soluble if the solute-solvent forces are as strong or stronger than the forces that must be broken • This gives rise to the “like dissolves like” principle • Polar solutes dissolve in polar solvents • The dipole-dipole solute-solvent forces are strong • Non-polar solutes dissolve in non-polar solvents • The LDF solute-solvent forces are similar to the solute-solute and solvent-solvent LDF

Metallic Bonding • The forces holding together metals is quite different from what we have seen • The high energy orbitals (often d) overlap creating a shared molecular orbital across the whole sample of metal • The upper electrons of all atoms are free to flow throughout the whole sample

Solid Types • There are several different ways that compounds can form solids • Some are held together by intermolecular forces while some are held together by bonds • And they all have different properties • Ionic crystals • Ionic compounds form crystal lattices where the cations are surrounded by several anions (and vice versa) • These are held together by ion-ion attractions and are very strong

Ionic Crystal Lattice • These have very high boiling points due to the strong attractions • They are also very brittle because every ion is firmly held in place

Molecular Crystal • Molecular compounds can be held together by intermolecular forces • The molecules are usually held firmly in place • Boiling points are relatively low due to weak attractions • Solids are usually brittle

Metals • As we have seen, metals have ions bound together in a sea of electrons • This allows them to be strong • This allows them to be malleable since atoms are not held in place by uni-directional forces

Covalent Networks • Some molecular compounds are held together by covalent bonds • This makes very strong structures • Atoms are held in place by strong covalent bonds • Molecules have high BP and low volatility • Solids are brittle • Silicon dioxide (sand) is a good example

Carbon Lattices • The hybridization of carbon can produce interesting effects in its solid lattices • Diamond has only sp3 carbon atoms • Each atom is covalently bonded to four others • This creates an extremely rigid structure

Graphite • Graphite is composed of sp2 carbon atoms only • This produces a flat sheet • The p-orbitals all overlap on top forming a delocalized surface across the sheet • Graphite conducts electricity because of this • Several graphite sheets are held together by LDF

Carbon Nanotubes • The graphite sheets can be rolled up into tubes • These are called carbon nanotubes • They are being implemented as the electrical components of the future

Bonding, Intermolecular Forces, and Solids