CHAPTER 4B MOLECULAR SHAPE AND POLARITY.pdf

MOLECULAR SHAPE AND POLARITY CHAPTER 4B This Photo by Unknown Author is licensed under CC BY-SA OUTLINE  Molecular shape – Introduction  Polar and nonpolar molecules  Apply the valence-shell electron-pair repulsion (VSEPR) model to predict the shape of a molecule or polyato...

MOLECULAR SHAPE AND POLARITY CHAPTER 4B This Photo by Unknown Author is licensed under CC BY-SA OUTLINE  Molecular shape – Introduction  Polar and nonpolar molecules  Apply the valence-shell electron-pair repulsion (VSEPR) model to predict the shape of a molecule or polyatomic ion.  Predicts deviations from ideal bond angles in structures based on the presence of lone pairs on a central atom.  Assess whether a molecule will have a dipole moment.  Adoption of dipole moment to determine the polarity of a bond and molecules Molecular shape: Introduction  Molecular shape shows the 3- dimensional arrangement of atoms in a molecule.  Predicted by using Valence Shell Electron Pair Repulsion (VSEPR) theory VSEPR The Valence-Shell Electron Repulsion theory states that: The valence electron pairs around the central atom are oriented as far apart as possible to minimize the repulsion between them. STRENGTH OF REPULSION The Repulsion May Occur Either Between: a) bonding pair & another bonding pair b) bonding pair & lone pairs c) lone pair & another lone pairs  Strength of Repulsion The order of repulsive force is: Lone pair-lone > Lone pair-bonding > Bonding pair-bonding pair repulsion pair repulsion pair repulsion Stronger to weaker repulsion Note: The electron pairs repulsion will determine the orientation of atoms in space  Comparison of Bond Angle in CH4, NH3 and H2O Electrons in a bond are held by the attractive forces exerted by the nuclei of the two bonded atoms therefore, they take less space of repulsion. Lone- pair electrons in a molecule occupy more space; therefore, they experience greater repulsion from neighboring lone pairs and o 104.5o bonding pairs 107.3 109.5o SHAPE OF A MOLECULE Basic shapes are based on the repulsion between the bonding pairs. Steps to determine the molecular shape : Step 1 Draw Lewis structure of the molecule Step 2 Consider the number of bonding pairs Step 3 Place bonding pairs as far as possible to minimize repulsion. A. Molecules with 2 bonding pairs Shape : Example: BeCl2 Lewis structure Be : 2e 180° 2Cl :14e Total : 16 e.... : Cl Be.. Cl:.. Linear B. Molecules with 3 bonding-pairs Example: BCl3 Repulsive forces between pairs are the same Lewis structure B: 3e 3Cl : 21e 120° Total: 24e : Cl........ : Cl.. B Cl :.. Trigonal Planar C. Molecules with 4 bonding pairs Equal repulsion between bonding Example: CH4 pairs – equal angle Lewis structure H 109.5° H C H H Tetrahedral D. Molecules with 5 bonding pairs Example: PCl5 Shape: Lewis structure 90° : Cl...... 120° Cl: P.. Cl.. Cl :.. Cl.... : Trigonal bipyramidal E. Molecules with 6 bonding pairs Example: SF6 Lewis structure S : 6e 6F : 42e 90o Total : 48e 90o F F F S F F F Octahedral 2 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB2 2 0 180° Linear Beryllium Chloride © McGraw-Hill Education. 10-17 3 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding pairs lone pairs AB3 3 0 120° Trigonal planar Boron Trifluoride © McGraw-Hill Education. 10-19 4 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB4 4 0 109.5o Tetrahedral Methane © McGraw-Hill Education. 10-21 5 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB5 5 0 90° 120° Trigonal bipyramidal Phosphorus Pentachloride Jump to long description © McGraw-Hill Education. 10-23 6 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding pairs lone pairs AB6 6 0 90° 90° Octahedral Sulfur Hexafluoride © McGraw-Hill Education. 10-25  Effect of lone pairs on molecular shape The geometries can be predicted using VSEPR. Determined by the repulsions of electron pairs in the valence shell of the central atoms. Lone pair-lone > Lone pair-bonding > Bonding pair-bonding pair repulsion pair repulsion pair repulsion Stronger to weaker repulsion Shape of molecules which the central atom has one or more lone pairs Class of Number of Number of Shape molecules bonding lone pairs pairs AB2E 2 1 Bent / V-shaped Bond angle : < 120o 4 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding pairs lone pairs AB3E 3 1 Trigonal pyramidal Bond angle : < 109.5o 4 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB2E2 2 2 Bent / V-shaped Bond angle : < 109.5o 5 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB4E 4 1 Distorted tetrahedral (see-saw) Bond angle : < 90o  5 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB3E2 3 2 T-shaped Bond angle : < 90o 31  5 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB2E3 2 3 Linear Bond angle : 180o 32  6 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding lone pairs pairs AB5E 5 1 Square pyramidal Bond angle :90o and 180o 33 6 electron pairs in the valence shell of central atom: Class of Number of Number of Shape molecules bonding pairs lone pairs AB4E2 4 2 Square planar Bond angle : 90o VSEPR: 3 Electron Groups # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom trigonal trigonal AB3 3 0 Planar planar trigonal AB2E 2 1 bent planar © McGraw-Hill Education. 10-35 VSEPR: 4 Electron Groups (1 of 2) # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom AB4 4 0 tetrahedral tetrahedral trigonal AB3E 3 1 tetrahedral pyramidal © McGraw-Hill Education. 10-36 VSEPR: 4 Electron Groups (2 of 2) # of atoms # lone Arrangement Molecular Class bonded to pairs on central of electron Geometry central atom atom pairs AB4 4 0 tetrahedral tetrahedral trigonal AB3E 3 1 tetrahedral pyramidal AB2E2 2 2 tetrahedral bent © McGraw-Hill Education. 10-37 VSEPR: 5 Electron Groups (1 of 3) # of atoms # lone Arrangement of Molecular Class bonded to central pairs on central electron pairs Geometry atom atom trigonal trigonal AB5 5 0 bipyramidal bipyramidal trigonal distorted AB4E 4 1 bipyramidal tetrahedron © McGraw-Hill Education. 10-38 VSEPR: 5 Electron Groups (2 of 3) # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom trigonal trigonal AB5 5 0 bipyramidal bipyramidal trigonal distorted AB4E 4 1 bipyramidal tetrahedron trigonal AB3E2 3 2 T- shaped bipyramidal © McGraw-Hill Education. 10-39 VSEPR: 5 Electron Groups (3 of 3) # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom trigonal trigonal AB5 5 0 bipyramidal bipyramidal trigonal distorted AB4E 4 1 bipyramidal tetrahedron trigonal AB3E2 3 2 T-shaped bipyramidal trigonal AB2E3 2 3 linear bipyramidal © McGraw-Hill Education. 10-40 VSEPR: 6 Electron Groups (1 of 2) # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom AB6 6 0 octahedral octahedral square AB5E 5 1 octahedral pyramidal © McGraw-Hill Education. 10-41 VSEPR: 6 Electron Groups (2 of 2) # of atoms # lone Arrangement of Molecular Class bonded to pairs on electron pairs Geometry central atom central atom AB6 6 0 octahedral octahedral square AB5E 5 1 octahedral pyramidal square AB4E2 4 2 octahedral planar © McGraw-Hill Education. 10-42 POLAR & NONPOLAR MOLECULES FYI:POLAR AND NONPOLAR MOLECULES A quantitative measure of the polarity of a bond is its dipole moment ( µ ). µ = Qd µ Where : µ = dipole moment Q = the product of the charge from electronegativity d= distance between the charges Covalent bonds between different atoms have different bond lengths. Covalent bonds can be polar or nonpolar, depending on the electronegativity difference between the atoms involved.  Polarity of HF  Hydrogen fluoride is a covalent molecule with a polar bond.  F atom is more electronegative than H atom, so the electron density will shift from H to F.  The symbol of the shifted electron can be represented by a crossed arrow to indicate the direction of the shift. H F δ + : partial positive charge δ - : partial negative charge Youtube: https://youtu.be/s1f7DzYF0jI  Diatomic molecules containing atoms of different elements have dipole moments and are called polar molecules.  Diatomic molecules containing atoms of the same element do not have dipole moments and are called nonpolar molecules.  For polyatomic molecules, the polarity of the bond and the molecular geometry determine whether there is a dipole moment. NON-POLAR MOLECULES Symmetrical molecules Basic molecular shape with the same terminal atom (CF4, CH4) Molecules with lone pairs linear and square planar with the same terminal atom (ICl¯, XeCl4) Retrieved from https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_CHEM_330_-_Adventures_in_Chemistry_%28Alviar- Agnew%29/04%3A_Chemical_Bonds/4.12%3A_Shapes_and_Properties-_Polar_and_Nonpolar_Molecules POLAR MOLECULES Non-symmetrical molecules Basic molecules with different terminal atom (CHCl3) Molecules with lone pairs except linear and square planar (NH3, H2O) Retrieved from https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_CHEM_330_-_Adventures_in_Chemistry_%28Alviar- Agnew%29/04%3A_Chemical_Bonds/4.12%3A_Shapes_and_Properties-_Polar_and_Nonpolar_Molecules  Steps to Identify Polar Molecules https://sciencenotes.org/polar-and-nonpolar-molecules/ Draw the Lewis structure Identify how many things (bond and lone pair) Draw VSEPR shape/geometry from Lewis's structure Visualize the net dipole moment If the net dipole moment direction cancel out Bonding electrons are evenly distributed in nonpolar each other, it is non-polar. Otherwise, it is polar molecules, but unevenly distributed in polar molecules.  EX 1: Predict the polarity of the following molecules: i. Carbon dioxide, CO2 ii.Carbon tetrachloride, CCl4 iii.Chloromethane, CH3Cl iv.Ammonia, NH3 REMEMBER! Factors that affected the polarity of molecules :  molecular geometry  electronegativity of the bonded atoms. Predict the polarity of the following molecules: i. Carbon dioxide, CO2 ii. Carbon tetrachloride, CCl4 iii.Chloromethane, CH3Cl iv. Ammonia, NH3  (a) Carbon dioxide, CO2 - molecular geometry : linear - oxygen is more electronegative than carbon, - Dipole moment can cancel each other - has no net dipole moment (µ = 0) - therefore, CO2 is a nonpolar molecule.  (b) Carbon tetrachloride, CCl4 - molecular geometry : tetrahedral - Chlorine is more electronegative than carbon, - Dipole moment can cancell each other - has no net dipole moment (µ = 0) - therefore, CCl4 is a nonpolar molecule.  ( c) Chloromethane, CH3Cl - molecular geometry : tetrahedral - Cl is more electronegative than C, C is more electronegative than H - Dipole moment cannot cancel each other - has a net dipole moment (µ ≠ 0) - therefore CH3Cl is a polar molecule.  (d ) Ammonia, NH3 - molecular geometry : trigonal pyramidal - N is more electronegative than H, - Dipole moment cannot cancell each other - has a net dipole moment (µ ≠ 0) - therefore NH3 is a polar molecule.  EX 2: Predict whether each of the following molecules has a dipole moment: a BrCl b BF3 (trigonal planar) c  CH2Cl2 (tetrahedral) Keep in mind that the dipole moment of a molecule depends on both the difference in electronegativities of the elements present and its geometry. A molecule can have polar bonds (if the bonded atoms have different electronegativities), but it may not possess a dipole moment if it has a highly symmetrical geometry. © McGraw-Hill Education. 10-60 Solution a) Because bromine chloride is diatomic, it has a linear geometry. Chlorine is more electronegative than bromine, so BrCl is polar with chlorine at the negative end Thus, the molecule does have a dipole moment. In fact, all diatomic molecules containing different elements possess a dipole moment. © McGraw-Hill Education. 10-61 b) Because fluorine is more electronegative than boron, each B−F bond in BF3 (boron trifluoride) is polar and the three bond moments are equal. However, the symmetry of a trigonal planar shape means that the three bond moments exactly cancel one another: An analogy is an object that is pulled in the directions shown by the three bond moments. If the forces are equal, the object will not move. Consequently, BF3 has no dipole moment; it is a nonpolar molecule. © McGraw-Hill Education. 10-62  c) The Lewis structure of CH2Cl2 (methylene chloride) is This molecule is similar to CH4 in that it has an overall tetrahedral shape. However, because not all the bonds are identical, there are three different bond angles: HCH, HCCl, and ClCCl. These bond angles are close to, but not equal to, 109.5°. © McGraw-Hill Education. 10-63 Because chlorine is more electronegative than carbon, which is more electronegative than hydrogen, the bond moments do not cancel, and the molecule possesses a dipole moment: Thus, CH2Cl2 is a polar molecule. © McGraw-Hill Education. 10-64