Wednesday, November 11, 2009

A Comparison of Ionic and Molecular Compounds

• force that holds ions together in ionic compounds is a very strong overall attraction between positive and negative charges
• in molecular compound, the covalent bonds of the atoms making up each molecule are also strong
• the forces of attraction between molecules are much weaker than the forces among formula units in ionic bonding which gives rise to different properties in the two types of compounds
• for ionic compounds--- high melting and boiling points, hard but brittle
• ionic compounds are hard but brittle because in an ionic crystal even a slight shift of one row of ions relative to another causes a large buildup of repulsive forces that make it difficult for one layer to move relative to another layer; but if one layer is moved the repulsive forces make the layers part completely causing ionic compounds to be brittle
• in the solid state ions cannot move so the compounds are not eletrical conductors
• in the molten state ionic compounds are eletrical conductors because the ions can move freely to carry eletrical current
• many ionic compounds are soluble in water
• when they are dissolved in water they do conduct eletrical current
• other ionic compounds do not dissolve in water because the attractions between water molecules and the ions cannot overcome the attractions between the ions
• polyatomic ion= a charged group of covalently bonded atoms
• polyatomic ions have characteristics of both molecular and ionic compounds
• the charge of a polyatomic ion results from either an excess or shortage of electrons
• use formal charge with polyatomic ions to figure out the most stable lewis structure
• in a polyatomic ion's lewis structure, often times brackets are drawn around the entire structure and the charge of the polyatomic ion is written on the outside of the brackets

Metallic Bonding

• chemical bonding is diferent in metals than it is in ionic, molecular, or covalent-network compounds
• thus they have special properties
• excellent eletrical conductors in solid state, strong reflectors and absorbers of light, luster
• metallic bonding= the chemical bonding that results form the attraction between maetal atoms and the surrounding sea of electrons
• sea of electrons----> in a metal vacant orbitals in atoms' outer energy levels overlap and this allows the outer electrons of the atoms to roam freely throughout the entire metal; the electrons are delocalized which means that they don't belong to any one atom but move freely about the metal's network of empty atomic orbitals; these mobile electrons form a sea of electrons which are packed in a crystal lattice
• malleability= the ability of a substance to be hammered or beaten into thin sheets
• ductility= the ability of a substance to be drawn, pulled, or extruded through a small opening to produce a wire
• ductility of metal is allowed because metallic bonding is the same in all directions throughout the solid which allows, when struck, one plane of atoms to slide past another without encountering any resistance

quiz tomorrow---> be prepared and read ahead

Dr. B might be posting on this blog tonight

Finally, do not forget to begin studying the tables that are in the next chapter so you won't have to wait till the night before to learn them all