Chapter 7: Properties of Ionic Covalent and Metal Materials Explanation: The ominous, dark shapes haunting the left side of the Sun are coronal holes -- low density regions extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. These coronal holes, however, have just moved into view near the Sun's equator, and particles escaping them have already caused notable aurora here on Earth. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on March 9 was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory. Chapter 7: Properties of Ionic Covalent and Metal Materials

Ions (Cations & Anions) Two or more nonmetal atoms Types of Atoms Ionic Compounds: Ions (Cations & Anions) Covalent (Molecular Compounds): Two or more nonmetal atoms Metallic Solids: Only metal atoms

Free moving valence electrons Bond formation Ionic Compounds: Transfer of electrons Form Ionic bonds Covalent (Molecular Compounds): Sharing of electrons Form Covalent bonds Metallic Solids: Free moving valence electrons Bond formation

Properties At room temperature they are a crystalline solid. Ionic Compounds: At room temperature they are a crystalline solid. Hard & Brittle, High melting point Covalent (Molecular Compounds): At room temperature they can be a low-melting pt solid, liquid or gas. Usually soft & low melting point

Properties Soft to very hard solids Why? Metallic Solids: Soft to very hard solids Why? Strength increases with an increase in # of e- available for bonding. Low to very high melting points Ductile: Metal can be drawn into thin wires Malleable: Metal can be hammered into different shapes

Compound Arrangement Three dimensional pattern Ionic Compound Three dimensional pattern How is the 3D pattern created? Opposite charged ions form close-packed layers. Layers stack together to make a regular repeating 3D pattern called a: Unit cell Closely packed layers will keep stacking to form a: Crystal

An Ion Coordination Number How many Na+ surround Cl-? Is the # of opposite- charged ions that surround an ion in a crystal 6 Na surround a chlorine ion. How many Na+ surround Cl-?

Compound Arrangement Molecular Compound Consist of atoms (nonmetals) which form molecules. The bonds between molecules are weak forces. Weak forces Water molecules

vs. Ionic Compounds Molecular Compounds

How are metallic bonds formed? Compound Arrangement Metallic Solid Consist of closely packed cations. How are metallic bonds formed? They are formed from the attraction of free-moving valence electrons for the cations These electrons bind the cations together to create a metallic solid.

Electron cloud e- e- Form a bond Consist entirely of metal atoms. Typically hexagonal close packed, cubic close packed or body-centered cubic structures. These have coordination numbers of either 12 or 8. Bonding is due to valence electrons which are delocalized throughout the entire solid Bonding is stronger than simple dispersion forces, but there are insufficient electrons to form ordinary covalent bonds. The strength of the bonding increases with the number of electrons available for bonding Delocalization of electrons is the physical basis for the ability of metals to carry electrical current (electrons are free to move about the metal structure) The nucleus and inner core of electrons are in a "sea" of delocalized, mobile valence electrons Form a bond

Type of Bond Force & Strength Ionic Compounds: Electrostatic forces Very Strong Covalent (Molecular Compounds): Intermolecular forces Weak The force of attraction between atoms in metals, such as copper and aluminum, or alloys, such as brass and bronze, are metallic bonds. Dispersion forces, Dipole-dipole, H-bonding Metallic Solids: A “sea” of Valence Electrons Strong Strength

Conductors Conductors Two ways ionic compounds conduct electricity: Good conductors of electricity (dissolved ionic compounds are electrolytes) Two ways ionic compounds conduct electricity: Conduct an electric current in the melted state. Conduct electricity when the compound is dissolved in water. Conductors

Conductors How do ionic compounds conduct electricity? Ionic Compounds cont… How do ionic compounds conduct electricity? Bonds break (melt) or dissociate (dissolve). The ions separate and are free to move around. The ion movement produces a flow of electricity! Apply voltage allowing the cations move to one electrode & the anions to the other.

Conductors Poor conductors Why? Molecular Compounds: Intermolecular bonds will break easily, but not form ions. Covalent bonds (holding the molecule together) do not break easily. Molecular compounds do not readily disassociate in water. Molecular compounds do not contain ions that can conduct electricity.

Good conductors of electricity Metallic Solids: Good conductors of electricity Why? Free-moving valence electrons Good conductors of heat In addition to being malleable and ductile, they are also very good conductors of electricity. Electricity depends upon the flow of electrons. Whenever electrons can flow easily through a structure, then that structure is said to be a good electrical conductor. Clearly, the very fluid nature of the electron sea allows it to be a very good electrical conductor. Because of this quality, metals are usually used in the electronics industry Why? Free-moving valence electrons

Good conductors of heat & electricity.

Network Solids Network Solid Consist of atoms held together by large network of covalent bonds. Each atom is covalently bonded in a large chain or network. There are NO molecules in a network solid, only atoms bonded together. Covalent solids, such as diamond, form crystals that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds. Each carbon atom in diamond is covalently bound to four other carbon atoms oriented toward the corners of a tetrahedron, as shown in the figure below. Because all of the bonds in this structure are equally strong, covalent solids are often very hard and they are notoriously difficult to melt. Diamond is the hardest natural substance and it melts at 3550°C Bonds are very strong. Example: Diamond Network Solid

Metallic solids can arrange its bonds to form 3 orderly patterns: Body-centered cubic Face-centered cubic

Why are metallic solids the simplest crystalline solid? Hexagonal close-packed Close packing geometries Why are metallic solids the simplest crystalline solid?