Bonding-Notes9

Bonding-Notes9 - Chem 59-250 Ionic Bonding Whereas the term...

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Chem 59-250 Ionic Bonding Whereas the term covalent implies sharing of electrons between atoms, the term ionic indicates that electrons are taken from one atom by another. The nature of ionic bonding is very different than that of covalent bonding and must be considered using different approaches. Some aspects to remember: 1. Electronegative atoms will generally gain enough electrons to fill their valence shell and more electropositive atoms will lose enough electrons to empty their valence shell. e.g. Na: [Ne]3s 1 Na + : [Ne] Ca: [Ar]4s 2 Ca +2 : [Ar] Cl: [Ne]3s 2 3p 5 Cl - : [Ar] O: [He]2s 2 2p 4 O -2 : [Ne] 2. Ions are considered to be spherical and their size is given by the ionic radii that have been defined for most elements (there is a table in the notes on Atomic Structure). The structures of the salts formed from ions is based on the close packing of spheres. 3. The cations and anions are held together by electrostatic attraction.
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Chem 59-250 Ionic Bonding Because electrostatic attraction is not directional in the same way as is covalent bonding, there are many more possible structural types. However, in the solid state, all ionic structures are based on infinite lattices of cations and anions . There are some important classes that are common and that you should be able to identify, including: CsCl NaCl Zinc Blende Fluorite Wurtzite And others…Fortunately, we can use the size of the ions to find out what kind of structure an ionic solid should adopt and we will use the structural arrangement to determine the energy that holds the solid together - the crystal lattice energy, U 0 . F Ca F
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Chem 59-250 Ionic Bonding Most ionic (and metal) structures are based on the “close packing” of spheres - meaning that the spheres are packed together so as to leave as little empty space as possible - this is because nature tries to avoid empty space . The two most common close packed arrangements are hexagonal close-packed (hcp) and cubic close packed (ccp). Both of these arrangements are composed of layers of close packed spheres however hcp differs from ccp in how the layers repeat (ABA vs. ABC). In both cases, the spheres occupy 74% of the available space. Because anions are usually bigger than cations, it is generally the anions that dominate the packing arrangement. Usually, the smaller cations will be found in the holes in the anionic lattice, which are named after the local symmetry of the hole (i.e. six equivalent anions around the hole makes it octahedral, four equivalent anions makes the hole tetrahedral). hcp ccp
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Chem 59-250 Ionic Bonding Some common arrangements for simple ionic salts: Cesium chloride structure 8:8 coordination Primitive Cubic (52% filled) e.g. CsCl, CsBr, CsI, CaS Rock Salt structure 6:6 coordination Face-centered cubic (fcc) e.g. NaCl, LiCl, MgO, AgCl Zinc Blende structure 4:4 coordination fcc e.g. ZnS, CuCl, GaP, InAs Wurtzite structure 4:4 coordination hcp e.g. ZnS, AlN, SiC, BeO
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Chem 59-250 Ionic Bonding
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This note was uploaded on 10/23/2010 for the course TE 2098 taught by Professor Michaljo during the Spring '10 term at Bloomsburg.

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Bonding-Notes9 - Chem 59-250 Ionic Bonding Whereas the term...

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