1 Octet Rule Exam 1 Study Guide a Noble gases have complete electron shells which make them very stable i Halogens are one electron away from filled energy levels so they are very reactive ii Elements want to be full to be the most stable iii The octet rule results in filled s and p orbitals iv The octet rule relates to drawing Lewis Dot structures there should be EIGHT electrons around each atom 2 Lewis Dot Structures a Example i NF3 1 Arrange the atoms so that there are 3 F s and 1 N the single N will be 2 Go look at the periodic group number of each element the central atom a N is group 5 b F is group 7 3 There are 3 F s so 3 x 7 21 a There is 1 N so 5 b 21 5 26 4 Draw 26 valence electrons Each line bond counts for 2 5 All of the elements want 8 around them 3 VSEPR Theory 6 a Shape of the molecules matters i DNA is right handed ii Amino acids are left handed iii Melting and boiling points are affected iv Etc b VSEPR theory helps to explain how the shapes are made i The outermost electrons their bonds and lone pairs repel each other ii Due to electron electron repulsion these regions are arranged as far apart as possible iii Think of balloons they have to spread out 1 2 Draw the Lewis Structure 3 Count the number of lone pairs on the central atom and number of atoms bonded to the central atom 4 Predict the geometry of the molecule 5 c Electronic Geometry d Molecular Geometry i Distribution of regions of high electron density around the central atom 1 For example tetrahedral is an electronic geometry i Arrangement of atoms around the central atom 1 For example trigonal pyramidal is molecular geometry 4 Dipole Moment a b Tell us about the charge separation in a molecule c The measure of net molecular polarity d Non polar molecule i Even distribution 1 e Polar molecule ii Dipole moments for al bonds cancel out i Not even ii One end has more electrons iii Think POLAR one POLE is bigger 1 1 2 3 4 5 iv Dipole moments for all bonds DON T cancel out 1 The molecule has a resting net dipole moment f Determining Polar and Non Polar i Are the following polar or non polar It is pointing towards the more electronegative Cl a b Polar a They are all pulling equally even no one wins b Cl is on all sides c Non polar a Although it does not look even and perfectly spread out the pull is to all the F s and they cancel out b Non polar a The line is straight but the colors are all different so it can t be equally pulling and even b Polar a The orange are even at the top but the green pulls is down and throws off the balance so it can t be even and pulling equally in all directions b Polar 6 7 8 a The colors are all green so equal pull BUT one of the green are pulling it up while the other green pull to the sides so it can t be evenly balanced b Polar a The green are pulling in each direction b Non polar a At first glance it looks to be the same shape as the non polar one above and it looks balanced and even b BUT the green are both on one side and the purple are on the other so therefore is can t be even because the pull isn t even g Summary c Polar i Bonds between different atoms have a dipole ii Dipole moments depend on distance and charge iii Bond dipoles sum to give molecular dipoles iv Molecules where dipoles cancel are non polar v Molecules where dipoles don t cancel are polar vi Molecular dipoles dictate molecular properties 5 Intramolecular forces Influence the chemical properties of the molecule a Exist within each molecule b c Types i Ionic covalent and metallic bonds a Exist between the molecules b c Responsible for the state of the system Influence the physical properties of the substance 6 Intermolecular Forces i Gas liquid solid d Bonds are much weaker e Types i Dipole dipole 1 The positive end of one is attracted to the negative end of the other and vice versa Larger dipole stronger attraction In solution they have both attractive and repulsive forces 2 3 4 Energy 5 25 kJ mol ii Ion dipole 1 The ion s attracted to the opposite charge on the polar molecule 2 Energy 40 600 kJ mol 3 Depends on ion size charge and dipole strength iii Instantaneous dipole Ion induced dipole 1 2 Dipole induced dipole 3 Instantaneous dipole induced dipole iv Polarizability INCREASES down the periodic table 1 2 DECREASED from left to right across the periodic table 3 Longer molecules are more easily polarized than compact molecules v Hydrogen bonding 1 Special dipole dipole interaction between the hydrogen atom in a polar N H O H or F H bond and a lone pair on O N or F atom 2 Hydrogen bonding molecules have a Strong permanent dipole b A hydrogen atom c An atom with lone pair electrons N F O 3 Forces between polar molecules that contain an H atom bonded to a small highly electronegative atom N O F with lone electron pairs 4 Formic acid and some other molecules HCOOH can form hydrogen f Example bonds in multiple ways i What types of intermolecular forces exist between the following pairs ii HBr and H2S 1 FIRST classify as ionic polar or nonpolar a HBr is polar b H2S is polar c d Dipole Dipole So they both have a dipole iii Cl2 and CBr4 1 Cl2 is non polar it is even 2 CBr4 is non polar it is even too 3 They are both non polar so it must be instantaneous that it happened 4 Instantaneous dipole induced dipole iv I2 and NO3 I2 is non polar it is evenly pulling 1 2 NO3 has a negative Ionic 3 There is no dipole here but there is ion so it must be induced 4 Ion induced dipole v NH3 and C6H6 1 NH3 is polar not evenly spread out 2 C6H6 is nonpolar all even with no dipole 3 So we have one dipole and one no dipole 4 Must be dipole induced dipole g Example i Which of the following can form hydrogen bonds with water 1 Look a b Does it have a lone pair Is it N O F ii CH3OCH3 iii CH4 iv F2 v HCOOH vi Na 1 CH4 is non polar so it cannot hydrogen bond 2 F2 can a little so you would not be wrong to choose it but it isn t the best answer 3 Na is not N O or F 4 Correct CH3OCH3 and HCOOH formic acid always can h Strengths of intermolecular forces i The …
View Full Document
Unlocking...