Ionic Compounds and Ionic Bonds
Compounds composed of cations and anions are called ionic compounds.
Ionic bonds – the electrostatic force that holds ions together in ionic compounds.
Notice how the ions formed have the stable electron configuration of the octet of a noble gas.
Formula unit – The lowest whole number ratio of ions in an ionic compound
The coordination number of an ion is the number of ions of opposite charge that surround the ion in a crystal.
Properties of Ionic Compounds
1. Usually a solid crystal at room temperature
2. Generally have a high melting point
(for example NaCl mp = 800 degrees C)
3. Can conduct an electric current when melted or dissolved in water
Bonding in Metals
• Metals are made of closely packed cations, not neutral atoms
• Valence electrons of metals can be modeled as a Sea of Electrons.
• Metallic bonds are the attraction of the free floating valence electrons for the cations
• High electrical conductivity – The large number of mobile electrons explains why metals have electrical conductivity several 100 times greater than a typical nonmetal. Ag is the best conductor.
More about metallic bonds
• Good conductor of heat – due to frequent collisions between the mobile electrons of metals
• Ductile and malleable – the sea of electrons acts like a flexible glue, holding the nuclei together, so metal crystals can be deformed without shattering
• Luster – most metals have a silvery white metallic color because they reflect light of all wavelengths. Because electrons are not restricted to a particular bond, they can absorb and re-emit light over a wide wavelength range.
More metal facts
• No metal dissolves in water, but the only liquid metal (at room T), mercury, dissolves many metals, forming solutions called amalgams. A Ag-Sn-Hg amalgam is used in filling teeth.)
• Lowest melting point for metals is Hg at -39 C, and the highest is for tungsten at 3410 C.
Metals are crystalline
Metals are arranged in very compact and orderly patterns
Hexagonal close-packed arrangement
Body-centered cubicEvery atom has 8 neighborsNa, K, Fe, Cr, W
Face-centered cubicEvery atom has 12 neighborsCu, Ag, Au, Al, Pb
Hexagonalclose-packedEvery atom has 12 Neighbors Mg, Zn, Cd
Alloys are mixtures of 2 or more elements, at least one of which is a metal.