Molecular Geometry. Bond length: the distance between two atoms held together by a chemical bond

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2 Molecular Geometry Bond length: the distance between two atoms held together by a chemical bond Bond length decreases as the number of bonds between two atoms increases. Single bond is the longest. Triple bond is the shortest. 2

3 Molecular Geometry Bond angle: the angle made by the lines joining the nuclei of the atoms in a molecule O o 3

4 Who cares about molecular shape? The shape of a molecule plays a very important role in determining its properties. Properties such as smell, taste, and proper targeting (of drugs) are all the result of molecular shape.

5 Molecular Shape Lewis structures don t show us how atoms in a molecule are arranged in 3-dimensional space. Could you have predicted the arrangement of atoms on the right from just seeing it s Lewis structure?

6 So how do we find the shape of a molecule? A useful model for predicting the shape of molecules is the

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8 Main Premise of VSEPR Theory Molecules will adopt a shape that is lowest in energy A low energy shape is one that minimizes the valence shell electron pair repulsion (VSEPR) between adjacent atoms

9 Atoms in a molecule try to spread out from one another as much as possible to reduce the like charge repulsion between their outer electrons.

10 methane, C 4 C You might think this is the farthest that the hydrogens can get away from each other But if you think in 3 dimensions, this shape actually causes less repulsion between the bonding pairs of electrons

11 The 5 Main Shapes: Molecules adopt a geometry (shape) that minimizes e e repulsions g this occurs when e- pairs are as far apart as possible.

12 ere are the STEPS for determining molecular geometry: 1. Draw a Lewis structure 2. Count the # of bonds and # of lone pairs around the central atom (these are called domains ) Single, Double and Triple bonds count as ONE domain Each lone pair counts as ONE domain 3. Use CART to determine shape Name of molecule shape is based on position of atoms (not domains)

13 What the heck is an electron domain? It s a region in space where electrons are likely to be So it s an area of high electron concentration A domain is wherever there is a bond or a lone pair around an atom Bonds = Bonding Domains Lone Pairs = Nonbonding Domains

14 ow many domains around each atom?

15 Remember the BIG PICTURE? These domains are all negatively charged so they want to spread out from each other as much as possible within a molecule. This minimizes like charge repulsion between electrons and represents the lowest possible energy state for the molecule.

16 Geometries

17 Let s take a look at some molecular shapes

18 VSEPR Example

19 VSEPR Example

20 VSEPR Example

21 NOTE: Lone pairs take up more space than bonding pairs and thus push atoms farther away from each other (decrease the predicted bond angles). Let s take a look at this phenomena

22 C methane, C less repulsion between the bonding pairs of electrons

23 .. ammonia C N N 3 You might predict the planar shape in the middle, but the lone pair forces the hydrogens downward

24 C.... N O.. water, 2 O

25 For tetrahedral e- arrangements: # lone pairs Bond angle Example C 4 N 3 2 O

26 Molecular Geometry Many of the molecules we have discussed have central atoms surrounded by 2 or more identical atoms: AB n where A = central atom B = outer atoms n = # of B atoms Examples: CO 2, 2 O, BF 3, N 3, CCl 4 26

27 C.... N O..

28 C N O

29 Now, Let s Combine VSEPR with Polarity..

30 Review If two atoms share electrons, but don t share them equally, we call it a polar covalent bond Electrons are pulled closer to the more electronegative atom One end of the molecule develops a partial negative charge because it has a higher electron density

31 Dipole Moments A molecule such as -Cl that has an area of partial positive charge and an area of partial negative charge is said to have a dipole moment The dipolar character is represented by an arrow pointing toward the (-) end of the molecule

32 Dipole Moments ALL diatomic molecules with a polar bond have a dipole moment Ex. -Cl SOME polyatomic molecules with polar bonds have a dipole moment Ex. water (does); carbon dioxide (does NOT) It depends on the molecular geometry!!!

33 Water is Polar It has polar -O bonds Its geometry is bent So the molecule is polar It has a dipole

34 BF 3 is Non-polar It has polar B-F bonds Its geometry is trigonal planar So the molecule is non-polar It does NOT have a dipole All the fluorines cancel

35 Polarity of Molecules Examples: 35

36 A parting thought Like dissolves Like Polar molecules dissolve in polar solvents Nonpolar molecules dissolve in nonpolar solvents Polar and nonpolar solutions will not mix Ex. Water (polar) and Oil (nonpolar)

37 ydrogen bonds Attractions between water molecules ydrogen bonding always involves hydrogen Affects water

38 ydrogen bonds Yes, hydrogen bonds are the strongest of the intermolecular forces Properties of water: Pulls water into droplets igh boiling point Surface tension

39 Network Solid Solids in which all the atoms are covalently bonded together. Very strong bonds Very high melting points Diamond is an example

40 ybridization When s and p orbitals overlap Types of bonds in sp orbitals sp = triple bond sp 2 = double bond sp 3 = single bond

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42 Pi ( ) Bonds Pi bonds are characterized by Side-to-side overlap. Electron density above and below the internuclear axis.