4.8 The Structure and Properties of Solids

What are the four types of solids?

• Ionic Solids:  metal and nonmetal

• Metallic Solid:  two metals

• Molecular Solid:  two nonmetals

• Covalent Network:  metalloids/carbon

Ionic Crystals

• Anions and cations are attracted together in CRYSTAL LATTICE

  • Each anion surrounded by cations and vice versa

Crystal Lattice

• Exists in three dimension

• Ionic substance held together by STRONG electrostatic attractions in all three dimensions

• Bonds are directional

• Lattice is composed of ions

• No molecules in ionic compounds, referred to as formula units

Physical Properties of Ionic Compounds

• hard, brittle crystalline solids

• relatively high melting and boiling points

• do not conduct electricity when solid

• conduct electricity when molten or in aqueous solution

• soluble in water

Metallic Crystals

• Atoms are packed closely together in 3 dimensions (CLOSE-PACKED LATTICE)

• Metals have low ionization energies and low energy unfilled orbitals 

• So the valence electrons become DELOCALIZED amongst (shared by) all the atoms

• No electrons belong to a particular atom, free to move throughout metal

Metallic Bonding

• Atoms have lost valence electrons, are positively charged (CATIONS)

• attraction of positive ions for mobile electrons provides force that holds structure together

• a lattice of positive ions filled by a mobile ‘sea’ of valence electrons

• Metals consist of closely packed atoms with free-moving valence electrons.

• The positively charged nuclei remain fixed while the electrons move between them.

• Metals are held together by metallic bonding, where electrons move freely between nuclei.

• “Sea of electrons” results in different physical properties of metallic crystals

Physical Properties of Metals

• attraction is between ions and mobile electrons, not between ions themselves

  • this means that layers of ions can slide past each other without breaking any bonds

  • means that metals are MALLEABLE and DUCTILE

• if atoms are different sizes (in ALLOYS), it is harder for layers to slide

  • alloys are usually harder than pure metals

• delocalized electrons are free to move from one side of lattice to the other and can carry an electric current

  • metals are good CONDUCTORS of both electricity and heat

• strength of bond between metals depends on how many electrons each atom shares

  • MP of Potassium is 337 K

  • MP of Calcium is 1123 K

  • MP of Scandium is 1703 K

• strength also depends on how far from the nucleus the sea of electrons are going down a group, the melting points will decrease

Table of Metallic Properties

Molecular Crystals

• The intermolecular forces (weaker than intramolecular forces) determine the structure and properties of molecular crystals.

• Types of intermolecular forces:

  • London dispersion forces

  • Dipole-dipole forces

  • Hydrogen bonding (especially in polar molecules)

• Polar molecules have both dipole-dipole and London dispersion forces.

• Non-polar molecules only have London dispersion forces.

• Properties of Molecular Crystals:

  • Due to weak intermolecular forces, molecular crystals tend to:

    • Have lower melting points.

    • Be less hard than ionic crystals.

  • Molecular crystals contain neutral molecules, so they do not conduct electricity well, either in pure form or in solution.

Properties of Molecular Crystals Table

Covalent Network Solids

• A covalent network crystal is a solid where covalent bonds form an interwoven network between atoms.

• Giant, three dimensional covalent structure

• The network of covalent bonds in these crystals contributes to their unique properties, such as hardness and strength.

• Look at the allotropes of carbon and at silicon dioxide

• ALLOTROPES:  different forms of an element that exist in the same physical state

Diamond

• Each carbon is joined to 4 other carbons in the tetrahedral shape

• Extremely strong structure!

• All intramolecular bonds

• Explains high MP, BP and exceptionally hard structure

Graphite

• comprises a giant covalent network in 2 dimensions

• each layer has C atoms bonded to 3 other C atoms

• each layer is very strong

• between the layers, only weak van der Waals forces hold the layers together

• distance between the sheets is quite large and the forces between very weak

  • so layers can slide over each other easily often used as lubricant

  • layers are easily rubbed off on paper - why it is used in ‘lead’ pencils

• there are delocalized electrons between layers that are free to move and so graphite can conduct electricity

Fullerenes

• approximately spherical molecule made up of five- and six-membered C ring

• C₆₀ resembles a soccer ball

• there is a small amount of delocalized electrons but not enough to strongly conduct electricity

• behaves as an electron deficient molecule and readily accepts electrons

• is a molecular molecule and can dissolve in some non-polar solvents

• lower MPs than diamond or graphite

Surface Tension

• surface tension on a liquid is like an elastic skin

• molecules within a liquid are attracted by molecules on all sides, but molecules right at the surface are only attracted downward and sideways

• in order to break the surface tension, the force must be greater than the intermolecular forces holding the molecules together

• stronger forces = higher surface tensions

• water has one of the highest surface tensions