Metallic, Ionic and Network Solids SCH4U1 Mr. Dvorsky

Ionic Solids Result from the reaction of a metal and a non-metal Because of the large electronegativity difference, electrons transfer from the metallic atom to the non-metallic atom and the ions that result attract each other. Thus ionic solids are held together by ionic bonds

Ionic Solids -can be considered an array of positive and negative ions -arranged such that every positive ion has a negative neighbour and vice versa. -there are no distinct molecules in an ionic solid. -the attraction that the oppositely charged ions have for one another gives the solid its stability

Ionic Solids These solids are hard, have high melting points due to the strong ionic bonds. Are brittle, and do not conduct electricity (no mobile electrons in them) When they dissolve in water, they dissociate into ions and therefore form solutions which conduct electricity (in liquid state the ions and electrons are free to move).

Ionic Solids When comparing the properties of ionic solids we need to consider the strength of the ionic bond. Strength of bond depends on: -1. atomic radii – the greater the distance between the ions = smaller force of attraction -2. the force of attraction between charged ions is proportional to the magnitude of charges on the ions (i.e. MgO has a higher melting point than NaCl because it has ions with greater charge on them)

Ionic Solids

Ionic solids are hard and brittle Hammering can cause like charges to become aligned. –when this happens the crystal breaks along the line at which like charges are repelling each other.

Metallic Solids

Metallic Solids In solid state, metals form metallic solids – composed of closely packed atoms held together by electrostatic interactions and free moving electrons Metals can vary in some properties: for example in softness or melting points. These differences can be explained by electron sea theory.

The electron sea theory Metals – closely packed atoms in which electrons are free to move. Nuclei remain at fixed points while electrons are mobile. Electrons don’t stick with one nucleus but instead move from one to the other – acts like a glue that holds the atoms together.

Theory explains some properties of metallic solids: have low ionization energies because loosely held electrons can move freely outside their valence orbitals The strong forces of attraction between positive nuclei and electrons results in closely packed metallic solids See Table 1 page 250

Molecular Solids A solid composed of individual molecules held together by IMF e.g. carbon dioxide, iodine, water, sulfur See Table 2, 250 for properties

Covalent Network Solids Consist of atoms covalently bonded to each other in a continous pattern. Can be 2D or 3D. Has no natural beginning or end. e.g. Diamond, Graphite (allotropes of carbon) -allotrope – one of two or more compounds consisting of the same element but having different physical properties. -can be more than one type of atom, e.g. SiO2

Most are comprised of carbon, silicon, or carbon and silicon compounds Most are comprised of carbon, silicon, or carbon and silicon compounds. –Group 14 of the periodic table. See figure 8 on page 252 – difference between quartz and glass Examples: graphite, diamonds, graphene, carbon nanotubes, buckyball Extremely hard, large amount of energy required to melt it.

In graphite, carbons arranged in trigonal planar. In diamond, carbons arranged in tetrahedron. So… In graphite not all electrons are involved as bonding pairs. Allows for some conductivity.

World’s strongest material: Lonsdaleite Forms when the Earth is struck by meteors made of graphite. Computer simulation of pure Lonsdaleite = 58 percent harder than diamond.

Semiconductors Covalent solids of elements such as silicon or germanium – conduct a small amount of electric current at room temp., but increases conductivity at higher temps. -this property separates them from metals -important part of transistors

Doping with arsenic. -semiconductors have full valence shell, arsenic has one more valence electron – increase in temp causes extra electron to jump to higher energy level and move from atom to atom easily [increased conductivity]. -this is an n-type semiconductor

Doping with boron or indium -indium and boron have one less valence electrons. The missing electron creates a hole which is filled by electrons from surrounding atoms. Increases conductivity. -this is a p-type semiconductor