Shapes of Molecules & Intermolecular Forces

Shapes of covalent molecules VSEPR – Valence Shell Electron Pair Repulsion Use this to predict shapes of covalent molecules Count number of bond pairs and lone pairs Predict shape

2 180 3 120 4 109.5 1 107 104.5 Linear BeCl2 BeH2 Triangular planar No. electron pairs: No. bond pairs: No. lone pairs: Shape molecule: Bond angle: Examples: 2 Linear 180 BeCl2 BeH2 3 Triangular planar 120 BCl3 AlH3 4 Tetrahedral 109.5 CH4 SiCl4 1 Pyramidal 107 NH3 PH3 V-shaped 104.5 H2O H2S

BeCl2 Be in Group II – 2 e-’s in outer shell Cl in Group VII – 7 e-’s in outer shell 2 bond pairs & 0 lone pairs Linear Cl Be Cl 1800

BCl3 B in Group III – 3 e-’s in outer shell Cl in Group VII – 7 e-’s in outer shell 3 bond pairs & 0 lone pairs Triangular planar Cl 1200 B Cl Cl

CH4 C in Group IV – 4 e-’s in outer shell H in Group I – 1 e- in outer shell 4 bond pairs & 0 lone pairs Tetrahedral H 109.50 H C H

NH3 N in Group V – 5 e-’s in outer shell H in Group I – 1 e- in outer shell 3 bond pairs & 1 lone pair Pyramidal N 1070 H H H

H2O H in Group I – 1 e- in outer shell O in Group VI – 6 e-’s in outer shell 2 bond pairs & 2 lone pairs V-shaped 104.50 O H H

Ionic & Covalent Characteristics Network of ions in a crystal Individual molecules Usually hard & brittle (metals) Usually soft (non-metals) High melting & boiling points (mainly solids) Low melting & boiling points (mainly liquids & gases) Solid at room T Liquid or gas at room T Conduct electricity when molten or dissolved in water Do not conduct electricity

Intramolecular bonding Occurs between ions in ionic compounds Hold ions together within molecules or crystal lattices Example – sodium chloride (NaCl)

Intermolecular forces Forces of attraction between covalent molecules 3 types – van der Waals, dipole-dipole & hydrogen bonds When these forces are strong molecules are attracted to each other and form solids or liquids

Van der Waals forces Weakest intermolecular force Exist between neutral atoms As electrons move within atoms temporary dipoles can occur

Opposite ends of these temporary dipoles attract each other Temporary so not very strong +H – H- - - +H – H- ¦ +H – H-

Dipole-dipole forces Stronger than van der Waals In polar molecules the positive pole of one molecule is attracted to the negative pole of another Permanent dipoles so quite strong

+H – Cl- - - +H – Cl- ¦ +H – Cl-

Hydrogen bonding Strongest intermolecular force When a H atom is bonded to small strongly electronegative atoms like N, O or F it gains a partial positive charge This positive H atom is then attracted to the negative poles in other molecules forming a hydrogen bond

Strength of intermolecular forces: H – C – C H+ -O – H+ - - - -O H H+ Strength of intermolecular forces: Van der Waals < dipole-dipole < hydrogen bonds

Boiling Points Boiling points are increased when there are intermolecular forces present Examples – (a) H2 and O2: Both molecules are non-polar so can only have van der Waals forces O2 has higher b.p. because its heavier +H – H+ -O == O- Mr = 2 Mr = 16

(b) C2H4 and HCHO: C2H4 is non-polar so can only have van der Waals forces HCHO has a C == O bond so is polar so can have dipole-dipole forces

+H H+ O- C C +H – C +H H+ H+ non-polar polar Dipole-dipole forces are stronger than van der Waals so HCHO will have a higher boiling point

(c) H2O and H2S: H2O has a polar O --- H bond and O is strongly electronegative so can form hydrogen bonds H2S has a polar S --- H bond but S is not very electronegative so can only form dipole-dipole forces and not hydrogen bonds

O S H H H H O = 3.5 S = 2.5 Hydrogen bonds are stronger than dipole-dipole forces so H2O will have a higher boiling point