10.1 Intermolecular Forces

• Remember that temperature is a measure of the random motions of the particles in a substance

• If energy is added, the motions of the molecules increase, and they eventually achieve the greater movement and disorder characteristic of liquid water

• Dipole-dipole forces are forces that act between a polar molecule

  • Molecules with dipole moments can attract each other electrostatically by lining up so that the positive and negative ends are close to each other

  • Dipole-dipole forces are typically only about 1% as strong as covalent or ionic bonds, and they rapidly become weaker as the distance between the dipoles increases

  • Strong dipole forces are seen among molecules in which hydrogen is bound to a highly electronegative atom, such as nitrogen, oxygen, or fluorine

  • Two factors account for the strengths of these interactions: the great polarity of the bond and the close approach of the dipoles

• Hydrogen bonding has a very important effect on physical properties

  • One factor is the relatively large electronegativity values of the lightest elements in each group,

  • The second factor is the small size of the first element of each group

  • Hydrogen bonding is also important in organic molecules

  • The alcohols methanol and ethanol have much higher boiling points than would be expected from their molar masses because of the polar O---H bonds in these molecules, which produce hydrogen bonding

• London dispersion forces: Forces that exist among noble gas atoms and non-polar molecules

• The dispersion forces in molecules with large atoms are quite significant and are often actually more important than dipole-dipole forces

10.2 The Liquid State

• Liquids exhibit many characteristics that help us understand their nature. We have already mentioned their low compressibility, lack of rigidity and high density compared with gases

• Many of the properties of liquids give us direct information about the forces that exist among the particles

• For a given volume, a sphere has a smaller surface area than any other shape

• Surface tension: The resistance of a liquid to an increase in its surface area

• A non-polar liquid such as mercury shows a convex meniscus

• The characteristic of a liquid, in which the cohesive forces, are stronger than the adhesive forces toward the glass

  • Another property of liquids strongly dependent on intermolecular forces is viscosity, a measure of a liquid’s resistance to flow

  • Molecular complexity leads to higher viscosity because very large molecules can become entangled with each other

• The liquid state has strong intermolecular forces and significant molecular motions

• As a starting point, a typical liquid might best be viewed as containing a large number of regions where the arrangements of the components are similar to those found in the solid, but with more disorder, and a smaller number of regions where holes are present

• We can use a relatively simple model for gases

10.3 An Introduction to Structures and Types of Solids

• Crystalline solids: Highly regular arrangement of their component

• Amorphous solids: Considerable disorder in their structures.

• The positions of the components in a crystalline solid are usually represented by a lattice

  • A particular lattice can be generated by repeating the unit cell in all three dimensions to form the extended structure

  • The smallest repeating unit of the lattice is called the unit cell

  • The structures of crystalline solids are most commonly determined by X-ray diffraction

  • Diffraction is due to constructive interference when the waves of parallel beams are in phase and to destructive interference when the waves are out of phase

  • Diffractometer: A computer-controlled instrument used for carrying out the X-ray analysis of crystals

• There are many different types of crystalline solids

  • When solid sodium chloride dissolves in the polar water, sodium and chloride ions are distributed throughout the resulting solution and are free to conduct electric current

  • Table sugar, on the other hand, is composed of neutral molecules that are dispersed throughout the water when the solid dissolves

• The third type of solid is represented by elements such as carbon, boron, silicon, and all metals

• These substances all have atoms at the lattice points that describe the structure of the solid

• In the Group 8A solids, the noble gas elements are attracted to each other with London dispersion forces

• The internal forces in a solid determine the properties of the solid

10.4 Structure and Bonding in Metals

• Metals are characterized by high thermal and electrical conductivity, malleability, and ductility

• The closest packing model for metallic crystals assumes that metal atoms are uniform, hard spheres.

• Knowing the net number of spheres (atoms) in a particular unit cell is important for many applications involving solids

• Examples of metals that form cubic closest packed solids are aluminum, iron, copper, cobalt, and nickel.

• Malleable: Can be pounded into thin sheets

• Ductile: Can be drawn to form a wire

• A related model that gives a more detailed view of the electron energies and motions is the band model, or molecular orbital (MO) model, for metals

  • In this model, the electrons are assumed to travel around the metal crystal in molecular orbitals formed from the valence atomic orbitals of the metal atoms

• The existence of empty molecular orbitals close in energy to filled molecular orbitals explains the thermal and electrical conductivity of metal crystals

• Metals conduct electricity and heat very efficiently because of the availability of highly mobile electrons

• Alloy: a substance that contains a mixture of elements and has metallic properties

  • In a substitutional alloy, some of the host metal atoms are replaced by other metal atoms of similar size.

  • An interstitial alloy is formed when some of the interstices (holes) in the closest packed metal structure are occupied by small atoms

10.5 Carbon and Silicon: Network Atomic Solids

• Many atomic solids contain strong directional covalent bonds to form a solid that might best be viewed as a “giant molecule”

• The two most common forms of carbon, diamond, and graphite, are typical network solids

• In diamond, the hardest naturally occurring substance, each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms to form a huge molecule

  • In the energy-level diagram for diamond, there is a large gap between the filled and the empty levels

• Silicon is also an important constituent of the compounds that make up the earth’s crust

  • Silicon compounds are fundamental to most of the rocks, sands, and soils

  • Like glass, ceramics are based on silicates, but with that, the resemblance ends

• We can enhance the conductivity of silicon by doping the crystal with an element such as boron, which has only three valence electrons, one less than silicon

• A ceramic contains two phases: minute crystals of silicates that are suspended in glassy cement.

• Ceramics seem an obvious choice for constructing jet and automobile engines in which the greatest fuel efficiencies are possible at very high temperatures

  • But, ceramics are brittle—they break rather than bend—which limits their usefulness

• Electrons must be in singly occupied molecular orbitals to conduct a current

• The p–n junction has revolutionized electronics; modern solid-state components contain p–n junctions in printed circuit

10.6 Molecular Solids

• There are many types of solids that contain discrete molecular units at each lattice position

• A common example is an ice, where the lattice positions are occupied by water molecules

• These substances are characterized by strong covalent bonding within the molecules but relatively weak forces between the molecules.

• The forces that exist among the molecules in a molecular solid depend on the nature of the molecules

• Water molecules are particularly well suited to interact with each other because each molecule has two polar O--H bonds and two lone pairs on the oxygen atom

  • This can lead to the association of four hydrogen atoms with each oxygen: two by covalent bonds and two by dipole forces

10.7 Ionic Solids

• Ionic solids are stable, high-melting substances held together by the strong electrostatic forces that exist between oppositely charged ions

• The larger ions, usually the anions, are packed in one of the closest packing arrangements

• The smaller cations fit into holes among the closest packed anions

• trigonal holes are formed by three spheres in the same layer

  • Tetrahedral holes are formed when a sphere sits in the dimple of three spheres in an adjacent layer

• Octahedral holes are formed between two sets of three spheres in adjoining layers of the closest packed structures

• Closest packed structures contain the same number of octahedral holes as packed spheres

• the most useful model for explaining the structures of these solids regards the ions as hard spheres that are packed to maximize attractions and minimize repulsions

10.8 Vapor Pressure and Changes of State

• One of the most important roles that water plays in our world is to act as a coolant

• The vaporization of water is crucial to the body’s temperature-control system through the evaporation of perspiration

  • Equilibrium: No further net change occurs in the amount of liquid or vapor because the two opposite processes exactly balance each other

  • There is no net change because the two opposite processes just balance each other

  • The pressure of the vapor present at equilibrium is called the vapor pressure of the liquid

  • When the system reaches equilibrium, the vapor pressure can be determined from the change in the height of the mercury column

  • Measurements of the vapor pressure for a given liquid at several temperatures show that vapor pressure increases significantly with temperature

• The melting and boiling points for a substance are determined by the vapor pressures of the solid and liquid states

• Sublimation: A process in which a substance goes directly from the solid to the gaseous state

• Heating curve: A plot of temperature versus time for a process where energy is added at a constant rate

• Ionic solids such as NaCl and NaF have very high melting points and enthalpies of fusion because of the strong ionic forces in these solids

• The changes of state are physical changes; although intermolecular forces have been overcome, no chemical bonds have been broken

10.9 Phase Diagrams

• A phase diagram is a convenient way of representing the phases of a substance as a function of temperature and pressure

• The phase diagram for water shows which state exists at a given temperature and pressure

  • It describes conditions and events in a closed system of the type represented, where no material can escape into the surroundings and no air is present

• The melting point of ice decreases as the external pressure increase

  • The maximum density of water occurs at 4°C; when liquid water freezes, its volume increases

  • When water freezes in a pipe or an engine block, it will expand and break the container.

  • This is why water pipes are insulated in cold climates and antifreeze is used in water-cooled engines.

  • The lower density of ice also means that ice formed on rivers and lakes will float, providing a layer of insulation that helps prevent bodies of water from freezing solid in the winter

• Liquid carbon dioxide released from the extinguisher into the environment at 1 atm immediately changes to a vapor

• Critical temperature: The temperature above which the vapor cannot be liquefied no matter the applied pressure

• Critical pressure: The pressure required to produce liquefaction at the critical temperature