Lecture 4 Chemistry p3 Molecular Shapes MCQ - Tagged PDF

Fundamentals of Chemistry: Part 3 The Shapes of Molecules Dr Sarah K Coleman Learning Goals 1) Understanding how Lewis Dot Structures relate to molecular geometry 2) Be able to predict molecular geometry of simple molecules by VSEPR 3) Understand electronegativity and apply...

Fundamentals of Chemistry: Part 3 The Shapes of Molecules Dr Sarah K Coleman Learning Goals 1) Understanding how Lewis Dot Structures relate to molecular geometry 2) Be able to predict molecular geometry of simple molecules by VSEPR 3) Understand electronegativity and apply to polar / non-polar bonds 4) Be able to apply bond polarity and molecular geometry to predict dipole of simple molecules Why is Molecular Shape Important? Shape influences chemical and physical properties; e.g. reactivity, melting points In biochemistry, pharmacology and medicine - molecules and drugs have to fit precisely into the enzyme or protein Shape prediction allows improved drug design and development PROTEIN DRUG fitting into binding site Enzymes have binding sites which recognise very precise shapes of molecules  Lewis Dot Structures vs Molecular Geometry Previously used Lewis Dot Structures (LDS) to understand bonding within molecules LDS does not give information about the 3-dimensional SHAPE of a molecule Information from LDS can be used to develop the GEOMETRY (3-d shape) of a molecule It is based on MINIMISING interactions between electron pairs This is called Valence Shell Electron Pair Repulsion (VSEPR) Methane as Lewis Methane has Dot Structure tetrahedral geometry Valence Shell Electron Pair Repulsion (VSEPR) Theory 1) Atoms and molecules want the lowest energy state – most stable 2) Electron groups around the central atom of a molecule 3) Have bonding pairs of electrons and lone pairs of electrons (from LDS) 4) Electrons are negatively charged - Valence shell electrons want to be as far apart as possible 5) The constraints of needing to be bonded and trying to be distant allows geometry to be predicted 6) Knowing the geometry allows molecular shapes and bond angles of molecules to be predicted. A molecule showing electron repulsion REMEMBER: electrons are negatively charged. Written as e- Central atom Other atoms Any questions: You can type in the chat function box during this live session (synchronous)? Or onto the Question Board in the Biochemistry Blackboard module and I will look at them later (asynchronous).  Some Terminology Definitions Co-ordination Number: number of bonded atoms Steric number (SN): Molecular geometry (MG): 3-dimensional arrangement of atoms in a molecule Electron geometry (EG): 3-D arrangement of bonding electron pairs and lone electron pairs around central atom Bond angle: the angle between the bonds How to Find the Shape of a Molecule Draw the Lewis Structure Count the No. of electron groups and identify them as bond pairs or as lone pairs Decide the electron-pair geometry e.g. linear, trigonal- planar, tetrahedral etc Put in positions of other atomic nuclei around the central atom Determine the molecular geometry...  Some Terminology Definitions Co-ordination Number: number of bonded atoms Steric number (SN): For Nitrogen (central atom) Co-ordination number is 3 Steric number is 4 Molecular geometry is Trigonal pyramidal Electron geometry is tetrahedral AMMONIA A = central atom; X = an atom; E = a lone pair of electrons  Worked example: Carbon dioxide, CO2 2 electron groups Lewis Dot Structure Electron geometry is: LINEAR For Carbon (central atom) 4 bonded pair Steric number is 2 0 lone pair Co-ordination number is Molecular geometry is: 2 LINEAR Bond angle is 180° Worked example: Methane, CH4 Lewis Dot Structure 4 electron groups Electron geometry is: TETRAHEDRAL 4 bonded pair 0 lone pair Steric number is 4 Molecular geometry is: Co-ordination number is TETRAHEDRAL 4 Bond angle is 109.5° Worked example: Carbonate ion, CO3 2- Lewis Dot Structure 3 electron groups Electron geometry is: TRIAGONAL PLANAR 4 bonded pair 0 lone pair Steric number is 3 Molecular geometry is: Co-ordination number is TRIAGONAL PLANAR 3 Bond angle is 120°  Worked example: Water, H2O Lewis Dot structure 4 electron groups Electron geometry is: TETRAHEDRAL Steric number is 4 2 bonded pair Co-ordination number is 2 lone pair 2 Molecular geometry is: BENT Bond angle is