Ionic bonding. Characteristics of ionic compounds. The ionic to covalent continuum. Not believed at first, but got the 1903 Nobel prize

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1 Ionic bonding umany compounds can be thought of as a collection of ions (M n+, X n- ) held together electrostatically uthis idea arose out of experiments by Arrhenius looking at the conductivity of solutions prepared by dissolving ionic compounds in water Not believed at first, but got the 1903 Nobel prize Characteristics of ionic compounds umost simple ionic compounds tend to form hard and brittle crystals uthey usually have high melting points several hundred or thousand Kelvin» however, salts that are liquid at room temperature have been prepared using organic cations uwhen molten they conduct electricity umost dissolve in high polarity solvents to form conducting solutions The ionic to covalent continuum uin practice, no compound is truly ionic ucompounds containing elements with very different electronegativities tend to be more ionic 1

2 Ionic size ucations are always smaller than the parent atom and anions are always larger than their parent atoms outermost electrons in a cations experience a higher effective charge than the outer electron in the neutral atom would» Na 186 pm but Na pm outermost electrons in a anions experience a lower effective charge than the outer electron in the neutral atom would Effect of ion charge Periodic trends in size usimilar to those found for atoms uincrease down a group decrease across a period assuming the ion has the same charge 2

3 Trends in physical properties udecreasing ion size and increasing ion charge favor better binding of the solid (higher lattice energy) this tends to give increased melting and boiling points Polarization and covalency u Ionic compounds tend have a considerable covalent contribution to their bonding when they contain polarizing cations polarizing cations are cations capable of distorting the anion s electron cloud towards the cation Fajan s rules usmall highly charged cations are more polarizing ularge highly charged anions are more polarizable upolarization is favored for cations that do not have a noble gas electron configuration Ag +, Cu +, Zn 2+, Cd 2+, Hg 2+, Tl + etc. 3

4 Physical effect of covalency u Ionic solids with a significant covalent contribution to bonding show anomalous physical properties may not be water soluble AgCl, CuI etc. AlF 3 MP 1290 o C, AlI 3 MP 190 o C Hydration of ions uionic solids are usually soluble in water because the dipole on water interacts with the ion charges negative end of dipole coordinates to cation strength of interaction increases with decreasing cation size and increasing charge ustrong coordination may lead to the formation of hydrates [Al(OH 2 ) 6 ] 3+ 3Cl - Structures of ionic compounds uit is often convenient to think about the cations lying in holes (interstices) between arrays of anions utypically, assume ions are hard spheres uusually, a compound will adopt a structure that maximizes the number of anions around each cation without causing the anions to touch 4

5 Radius ratio rules uit is possible to predict the type of ion coordination that you will get if you know the sizes of the anions and cations Structures with simple cubic packing ua simple cubic array contains holes that are eight coordinate structures include CsCl and CaF 2 Structures with octahedral cation coordination uclose packed arrays of anions have both octahedral and tetrahedral interstices filling oct holes in a CCP array gives the NaCl structure filling oct holes in a HCP array gives the NiAs structure 5

6 The NaCl structure Tetrahedral coordination ustructures based on filling tetrahedral holes in close packed anion arrays are commonly found fill all tet sites in CCP array - ZnS zinc blende fill all tet sites in a HCP array - ZnS Wurtzite Violations of the radius ratio rules uradius ratio rules only work for ~2/3 known compounds ions are not really hard spheres covalent contribution to bonding can mess things up ionic radius varies with coordination number 6

7 The bond triangle uthere is a continuum of different bonding types 7