Molecular Shape and Polarity The Importance of Geometry in Determining Physical Properties

Structure Determines Properties It is important to understand structure in order to predict properties Structure can be predicted Predicting molecular structure begins with Lewis Dot Diagrams

Drawing Lewis Dot Structures First step: decide what atoms are bonded together – This is not always an easy choice For most binary covalent compounds and polyatomic ions the central atom will be the atom that occurs only once in the formula. When in doubt use the most symmetrical atom arrangement. Using CH 4 and CO 3 2- as examples H O O H C H and C H O

Lewis Dot Structures- Step 2 2. Count the total number of valence electrons in the ion or molecule – CH 4 has 8 valence electrons, 4 from Carbon and 1 from each of the four H atoms – CO 3 2- has 24 valence electrons, 4 from Carbon, 6 from each of the three Oxygen, plus 2 added to give the ion a charge of 2-

Lewis Dot Structures- Step 3 Place a pair of electrons (dots) between the central atom and each of the other atoms. H O O H C H and C H O

Lewis Dot Structures- Step 4 Distribute the remaining outer electrons so that each atom has a filled outer energy level, a filled octet (exceptions H only 2, B only 6) A complete Structure This structure does not work it needs 2e-

Lewis Dot Structures- Step 5 When there are not enough electrons to satisfy the octet rule, make double or triple bonds When there are extra electrons, place these on the central atom.

Check up from the Neck Up! Draw Lewis Structures for each of the following. 1.H 2 S 2.NH NO CO 5.CCl 4 6.HCOOH (one H bonds to C, One to O)

Answers

Valence Electron Pair Repulsion Valence electron pair charge clouds repel each other The electron pairs stay as far away from each other as possible to minimize repulsion This theory explains many of the shapes of molecules.

Demonstration Of Water Molecule What element is represented by the balloons? What does the fist represent? What does the string represent? What do you expect to occur? Why?

Molecular Shapes Are determined by electron repulsion The geometry or shape which allows the electrons to be the farthest away from each other is the shape of the compound The shape which minimizes the repulsive force between electrons will be formed Nonbonded pairs are more repulsive than bonded pairs so take up more “space” around the atom – This forces the bonded pairs a little closer than they would like Perfect tetrahedral angles – 109.5°, the two bonds in water are at ~ 105° the distortion of this angle is because of the nonbonded pair repulsive force.

Let’s Use These Molecules to Explore Molecular Structure Using the previous table, predict what shape you would expect for each of these molecular structures ?

Check Your Predictions! Tetrahedral Bent structure; 2 bonding domains, 2 unbound domains 3 Bonding Domains; Trigonal Planar structure 4 Bonding domains; Tetrahedral Structure

Multiple Bond Molecular Shapes Double and triple bonds form 1 bonding domain Carbon atom can form 4 chemical bonds with several arrangements – These can be single bonds in a tetrahedral arrangement (4 bonding Domains) – Can form 1 double bond and 2 single bonds having trigonal planar geometry (2 bonding domains, 1 nonbonded domain) – 2 bonding domains creates a linear structure -double bonds are arranged in a linear molecule -1 single bond and 1 triple bond

Pre-Lab: Model Molecular Shape 9.2 Purpose: To investigate the three-dimensional shape of molecules by building models Question: What molecular shapes will be formed because of electron repulsion in these molecules? Background information: The geometry or shape which allows the electrons to be the farthest away from each other is the shape of the compound The shape which minimizes the repulsive force between electrons will be formed Nonbonded pairs are more repulsive than bonded pairs so take up more “space” around the atom Students will complete the pre-lab table and construct their hypothesis

Polarity Most chemical bonds are not 100% ionic nor 100% covalent because of differing electronegativities. Unequal sharing creates polar molecules or polar covalent bonds – H-Cl is a polar bond; H Cl, an arrow is used to represent a polar bond, pointing toward the more electronegative element. – Not all polar bonds create polar molecules, geometry plays a role Symmetrical arrangement of polar bonds can cancel polar effect. Unsymmetrical arrangements can produce polar molecules

Polarity Unsymmetrical arrangements occur in water, creating a polar molecule Many Physical properties, such as melting and boiling points and solubility are affected by degree of polarity of molecules

Apply what we’ve learned about polarity. Determine the polarity of HCN and SOCl 2 – First draw the Lewis structures and identify the geometry – Then use electronegativity to determine polarity (Take out your handout for reference) – Draw the appropriate arrows and geometry

Answers H  C  N en H=2.1, en C=2.5, en N=3.0 Linear O  S  Cl en S=2.5, en Cl= 3.0, en O=3.5 Cl trigonal pyramid

Both Geometry and Polarity Determine Physical Properties