Lecture 14 Professor Hicks Inorganic Chemistry (CHE151) Shapes of Molecules The shape of most molecules is not flat as they are drawn on paper The three dimensional arrangement of atoms is called the Geometry 1
VSEPR Theory Valence Shell Electron Pair Repulsion Theory used to predict the geometry Groups of electrons repel each other stay as far apart as possible Resulting arrangement of atoms determines the geometry Using VSEPR to Determine Geometry Count the total number of VSEPR pairs, and how many are bonding pairs on the central atom Find that case of total VSEPR pairs and bonding VSEPR pairs in the reference table read off geometry Tips: 1) Each lone pair of electrons counts as one VSEPR electron pair 2) Each bonding region counts as one VSEPR electron pair regardless of whether it has single, double, or triple bonds O N O total 3 VSEPR electron pairs on N: 1 lone pair 1 single bond 1 double bond 2 bonding VSEPR electron pairs geometry = bent 2
3
CF 4 Tetrahedral geometry F F C F F 4 VSEPR bonding pairs All 4 are bonding pairs When a molecule has one central atom the geometry of that atom is also used to describe the overall shape of the molecule Bond angles are all 109.5 CO 2 Linear geometry O C O 2 VSEPR bonding pairs Both are bonding pairs OCO bond angle is 180 4
Molecular Shape aka Geometry vs Arrangement of Electron Pairs Two ways to describe shape 1) Way atoms arranged= Geometry aka Molecular Shape 2) That is not always the same as the Arrangement of Electron Pairs Remember lone pairs do not have atoms attached If no lone pairs both shapes same NH 3 H N H H 4 VSEPR bonding pairs 1 lone pair 3 bonding Bond angles are 109.5 Geometry is trigonal pyramidal Electron pair arrangement is tetrahedral Geometry and electron pair arrangement are different 5
Electronegativity Tendency to attract/hold electrons Scale that runs 0-4 Calculated using IE and EA values F has highest electronegativity= 4.0 attracts electrons the best Increases going right and up on PT Linus Pauling Metals have lower electronegativites tend to lose electrons Non-metals have higher electronegativites tend to gain electrons) High or low electronegativty more reactive Least reactive elements (besides noble gases) in middle PT 6
Electronegativity and bonding Covalent bond = two atoms with electronegativity difference 0 to 0.40 Polar covalent bond = two atoms with electronegativity difference 0.5 2.0 Ionic bonding two ions electronegativity difference 2.1 4.0 (metal and non-metal) Cl Na+ Cl - F H H C H H 7
C-H bonds are the basis of the definition of non-polar bonds Electronegativities 2.5 (C) and 2.1 (H) non-polar bond Little/no polarity in the bond Compounds with mostly C-H bonds are non-polar 9.37 Classify these bonds as ionic, polar covalent, or covalent, and give your reasons: (a) the CC bond in H 3 CCH 3 (b) the KI bond in KI (c) the NB bond in H 3 NBCl 3 (d) the ClO bond in ClO 2. 8
O Dipole Moment HF has a polar bond The two ends of HF are different HF has a Dipole Moment Molecules with a dipole moments line up like a compass - + - H F + each end of the molecule is attracted to the oppositely charged plate Dipole moment CO 2 has 2 polar bonds But they are arranged symmetrically Both ends look the same CO 2 does NOT have a dipole moment! - + + - C O - the molecule does NOT line up like a compass! all the pushing and pulling results no preferred orientation 9
Dipole moment CN - has a polar bond CN - has an overall charge CN does NOT have a dipole moment - N + C -1 the ion does not line up it moves towards oppositely charged plate Dipole moment H 2 O has 2 polar bonds H 2 O has a dipole moment the molecule lines up like a compass - - + H O - H + H + O + + each end of the molecule is attracted to the oppositely charged plate 10
Polar vs Non-polar Ionic compounds Molecules with a significant (polar bonds) dipole moment Polar water and water soluble compounds Molecules with little or no dipole moment, often lots C-H bonds Non-polar fats, oils, grease Polarity, Geometry and Dipole Moment Classify each of the following as having a significant dipole moment or not and being polar or non-polar compounds. a. Li 3 N b. HI c. HAt d. CS 2 (S-C-S) e. CO 2 (O-C-O) f. SCSe (S-C-Se) g. TeCO (Te-C-O) h. O 3 (O-O-O) i. NH 3 j. PH 3 k. H 2 Se 11
9.37 Classify these bonds as ionic, polar covalent, or covalent, and give your reasons: (a) the CC bond in H 3 CCH 3, (b) the KI bond in KI, (c) the NB bond in H 3 NBCl 3, (d) the ClO bond in ClO 2. 10.7 Predict the geometries of these species using the VSEPR method: (a) PCl 3, (b) CHCl 3, (c) SiH 4, (d) TeCl 4. 12
10.11 Describe the geometry around each of the three central atoms in the CH 3 COOH molecule. 10.18 The dipole moments of the hydrogen halides decrease from HF to HI (see Table 10.3). Explain this trend. 13
10.20 Does the molecule OCS have a higher or lower dipole moment than CS 2? 14