Electronegativity and Bond Polarity• Sharing of electrons to form covalent

bonds and transfer of electrons to form ionic bonds represent two ends of a bonding continuum.– Bonding between the two ends of the

bonding continuum is described using electronegativity and bond polarity.

Electronegativity

• Electronegativity is the attraction of an atom for the shared electrons in a covalent bond.– Electronegativities are not measured

quantities.– Electronegativities are assigned based on

factors such as atomic size, electron affinity, and ionization energy.

Electronegativity

• The higher the electronegativity value, the more likely an element will attract extra electron density during compound formation.– Electronegativities increase from left to right

across a period and from bottom to top for a group.

– Fluorine is the most electronegative element with an electronegativity of 4.0.

Electronegativity

• Electronegativity values for elements.– Electronegativities increase from left to right for periods.– Electronegativities increase from bottom to top for groups.

Bond Polarity

• Electron density is not shared equally when elements with different electronegativities bond.– More than half of the electron density is

associated with the more electronegative element.

Bond Polarity

• The more electronegative element experiences an increase in electron density and a partial negative charge.

• The less electronegative element experiences a decrease in electron density and a partial positive charge.

• The two points of positive and negative charge constitute a dipole.

Bond Polarity

• A bond along which a dipole exists is a polar bond.– Also referred to as a polar covalent bond since electrons

are still being shared.• The greater the electronegativity difference, the

more polar the bond.• When the electronegativity difference is zero, the

bond is classified as nonpolar covalent.• When the electronegativity difference exceeds 2.0,

the bond is classified as ionic.

Bond Polarity

• The formation of the polar covalent HF bond.– The more electronegative F has a partial negative charge.– The less electronegative H has a partial positive charge.

Bond Polarity

• Before bonding, the electron density around H and F is spherical.– The negatively charged electrons and the positively charged nucleus offset each other.

Bond Polarity

• When bonded, the more electronegative fluorine attracts shared electron density, creating a partial negative charge on F, and a partial positive charge on H.

Bond Polarity

• The partial negative charge on F in PTFE forces the carbon backbone into a highly ordered shape.– PTFE packs into a crystal array more

efficiently.– Efficient crystal packing results in a

higher melting temperature for Teflon.

Bond Polarity

• The large bond polarity along the C-F bond in PTFE forces the carbon backbone into a highly ordered shape.

• Polymers with less polar bonds are not as highly ordered and are more “branched”.

Bond Polarity

• The large bond polarity along the C-F bond in PTFE forces the carbon backbone into a highly ordered shape.

Bond Polarity

• Polymers with less polar bonds are not as highly ordered and are more “branched”.

Intermolecular Forces

• Intermolecular forces - the attractive and repulsive forces between molecules.

Forces Between Molecules

• The attractive and repulsive intermolecular forces are weak in comparison to bonding forces.– Intermolecular forces are largely

responsible for determining the structure and properties of condensed phases.

Dispersion Forces

• Dispersion forces are common to all molecules.– Also called London forces.– Also referred to as instantaneous dipole-

induced dipole forces.– Dipoles exist for two oppositely charged

points separated by some distance.

Dispersion Forces

• An instantaneous dipole occurs when a fluctuation in electron density for an atom or molecule produces a dipole.– Instantaneous dipoles are short lived and

constantly created.

Dispersion Forces

• An induced dipole is created when an external electric field forces a dipole to exist.– A permanent dipole or an instantaneous

dipole are the sources of the external electric field.

Dispersion Forces

• An external electric field distorts the electron density around an atom, inducing a dipole.

Dispersion Forces

• Molecules and atoms have symmetric charge distribution in the absence of an external electric field.

Dispersion Forces

• The negative side of an external electric field repels negatively charged electrons, creating an induced dipole.

Dispersion Forces

• Dispersion forces are incredibly weak.– The sum of dispersion forces over an

Avogadro’s number of atoms results in a significant amount of energy.

– Dispersion energy holds many liquids and solids together.

Dispersion Forces

• The strength of a dispersion force can be estimated from the polarizability of a molecule.– Polarizability is a measure of how susceptible a

molecules electron density is to external electric fields.

– Large molecules are more polarizable than smaller molecules and experience stronger dispersion forces.

Dipole-Dipole Forces

• Dipole-dipole forces are the attractive and repulsive forces for molecules with a permanent dipole.– The charge at the poles of a dipole is seldom

more than a fraction of the charge on an electron.– Molecules with stronger dipoles have stronger

dipole-dipole forces.– Dipole-dipole forces are typically stronger than

dispersion forces.

Dipole-Dipole Forces

• Dipole-dipole forces for 50 polar molecules.– At any given time, the number of attractive interactions is greater than the number of repulsive interactions.

Hydrogen Bonding

• Hydrogen bonds are a special case of dipole-dipole forces.– Hydrogen bonds are especially strong

compared to dipole-dipole forces.– Hydrogen bonds occur only in

compounds containing hydrogen bonded to the highly electronegative elements N, O, and F.

Intermolecular Forces

• Intermolecular forces are weak compared to the average covalent bond.

• The large number of intermolecular forces make intermolecular forces a key factor in determining the bulk properties for materials.

Intermolecular Forces

• Carbon atoms within a graphite sheet held together by covalent bonds.

• Intermolecular forces hold graphite sheets together.

Intermolecular Forces

• Carbon atoms within a graphite sheet held together by covalent bonds.

Intermolecular Forces

• Intermolecular forces holds graphite sheets together.

Condensed Phases - Liquids

• Liquids are also a condensed phase.– Particles in condensed phases are in

constant contact.– Every particle in a solid vibrates around

a fixed position.– Every particle in a liquid is free to

constantly move with respect to one another.

Vapor Pressure

• Vapor pressure - the gas phase pressure of a substance in equilibrium with the pure liquid in a pure substance.– Vapor pressure is a characteristic

property of a particular substance at a particular temperature.

Vapor Pressure

• There is a distribution of kinetic energies for liquids at a given temperature.– Liquid molecules at the surface of the liquid with

sufficient kinetic energy will pass into the gas phase.

– Vapor pressure increases with temperature.– Liquids with strong intermolecular forces have

lower vapor pressures.– Liquids with high vapor pressures are described

as volatile.

Vapor Pressure

• To measure the vapor pressure of a solid or a liquid, the system must reach equilibrium.– The equilibrium is a dynamic equilibrium.– The rate of evaporation equals the rate of

condensation.– The amount of gas and liquid does not appear

to change.

Vapor Pressure

• Liquids in closed containers will establish equilibrium with its vapor phase.

Vapor Pressure

• Nonvolatile liquids evaporate more slowly due to their stronger inter-molecular attractive forces and have low vapor pressures.

Vapor Pressure

• Volatile liquids evaporate more quickly due to their weaker inter-molecular attractive forces and have higher vapor pressures.

Boiling Point

• Liquids boil when the vapor pressure for a liquid equals external pressure.– Normal boiling point - temperature

where the vapor pressure of a liquid equals atmospheric pressure.

– The stronger the intermolecular forces in a liquid, the lower the vapor pressure, and the higher the boiling temperature.

Boiling Point

• The vapor pressure of water varies dramatically as a function of temperature.

Boiling Point

• Vapor pressures at 295 K for various substances.

• Substances with weaker attractive forces have higher vapor pressures and lower normal boiling points.

Surface Tension

• Liquid molecules at the surface of a liquid experience fewer attractive forces than the molecules in the bulk material.– Surface tension is a liquids response to the

imbalance in attractive forces.– Liquids form spherical shapes.– Spherical shapes have the greatest volumes with the

least amount of surface area.– Stronger attractive forces result in strong surface

tension.

Surface Tension

• Surface tension is the result of molecules at the surface of a liquid experiencing fewer intermolecular forces than liquid molecules inside the bulk liquid.

Surface Tension

• The interaction between a liquid and the surface of a solid depends on two types of attractive forces.– Cohesion forces are liquid-liquid interactions.– Adhesion forces are liquid-solid interactions.– The relative strengths of the two forces dictate

the shape of a liquids meniscus.

Surface Tension

• The meniscus is the curved shape a liquid makes in contact with a solid.– Strong adhesion forces and weaker cohesion

forces result in a concave meniscus.– Weak adhesion forces and stronger cohesion

forces result in a convex meniscus.

Surface Tension

• The concave meniscus for water results from stronger adhesive forces.

• The convex meniscus for mercury results from stronger cohesive forces.