Topic 3 - Chemical Bonding, Polarity, Molecular Shape & Crystal Packing PDF

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Topic 3: Chemical Bonding, Polarity, Molecular Shape & Crystal Packing Electronegativity  The ability of one bonded atom to attract shared electrons towards itself  The most electronegative elements are those found in the top right corner of the periodic table  The least electronegative are those at the bottom left.  Increases from left to right across each row and from bottom to top in each column  Metals are less electronegative than non-metals  No values for noble gases  The further separated the elements are, the more polar the bond between them Polar Covalent Bonds  If the electronegativities of two atoms in a molecule differ, this results in an uneven distribution of charge within the molecule.  POLAR = any molecule in which there is a charge separation

When the electronegativities varies between tow atoms, the more electronegative atom takes on a partial negative charge, while the less electronegative atom becomes partially positively charged.  If the polarity of one bond is not cancelled by another polar bond, a dipole results.  Dipole moment = is a measure of the size of the polarity. It’s a vector quantity possessing both a magnitude and direction Measuring a dipole Magnitude  Place the compound into a solution containing metal electrodes  When an electric field is applied the molecules align themselves within that field on the basis of their charge separation Direction  Determined by vector analysis if the molecular geometry and electronegativity of atoms is known Dipole:  Bonds between atoms metals and non-metals are ionic  Bonds between atoms in the P-block and with Hydrogen are covalent  Individual bonds of a molecule may be polar, but the molecule itself may not 

Molecular Geometry  Shapes of molecules affects their polarity an can influence their physical and chemical properties  Shape of molecule is critical for the function of proteins and enzymes VSEPR (Valence Shell Electron Pair Repulsion Theory)  VSEPR – determines the shape of a molecule  VSEPR takes into account all valence electrons (those in bonds and lone pairs)

Molecular Shapes

Non – VSEPR Shapes  Can only be used for S & P type electrons  Elements in the P, D & F block don’t always conform  Two molecular shapes that are not defined by VSEPR is shown below

Non-bonding electrons  Electron pairs in the outer shell can contribute to the shape of a molecule without being part of the bonds between atoms Why does water not have a liner shape?  Oxygen forms 2 bonds and has 2 lone pairs of electrons  Lone pairs are the strongest (most repulsive) – they require the most amount of space  The second strongest is a lone pair and a bond  Weakest – two bonds – Least repulsive - requires the least amount of space  Electronic shape – roughly tetrahedral  Molecular shape – bent (only shows position of atoms)

Repulsions  Strongest – Weakest repulsions Lonepair: lonepair > lonepair : bonding > bonding : bonding  Affects the distribution in 3D space about the central atom  The more lone pairs present – the greater their repulsions -> the narrower the bond pairs angle becomes.

Molecular Polarity  Polarity depends on electronegativity differences between atoms and on molecular shape  E.g. CO2  The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. CO2 is a linear molecule so the C-O dipole vectors cancel each other out. The net result is a nonpolar molecule.

Polarity and Molecular Shape Polar Molecules:  Any molecular that is asymmetric in 3D  Bent molecules  Trigonal pyramidal molecules  Tetrahedral molecules in which the substituents are non-identical  CH3Cl, CH2Cl2 and CHCl3 have a dipole moment (i.e. are polar). The arrowhead points towards the negative end of the dipole.

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E.g. H2O, the H-O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule in H2O but the O-H vectors do not cancel each other out. The net result is a polar molecule

Non-Polar molecules:  Symmetric linear molecule e.g. H2  Trigonal planar molecules in which the 3 groups off the central atom are the same  Tetrahedral molecules in which all 4 groups off the central atom are the same  Trigonal bipyramidal molecules in which the 5 groups off the central atom are the same  Octahedral molecules in which the 6 groups off the central atom are the same  CH4 and CCl4 do not have a dipole moment and are non-polar . The arrowhead points towards the negative end of the dipole.

Lattices Ionic Lattices  In a solid each atom sits in an identical site to the next one.  Unit cell = the repeating unit for the way molecules sit in a crystal  Lattice = an infinite array of atoms.  Lattices of covalently bonded atoms also exists

Both diamond and graphite (forms of carbon) exists as infinite lattices Tetrahedral = the bonding is equal in all directions e.g. diamond which is why it’s so hard Graphite Lattices  Graphite exists as layers of planar sheets of fused 6- membered rings.  Each C bonds with other C atoms in the plane.  The interaction between layers is weak, and they can slide over one another  The extended lattice of graphite and diamond account for the high melting (3727 C) and boiling points (4830 C) of C.  

Fullerenes  Fullerene are molecular substances, each C60 is a separate unit  This affects the physical properties e.g. C60 sublimes at 450*C while diamond and graphite melt at T>3500*C. Crystalline & Amorphous Solids  Crystalline solids have a regular repeating unit and as such are ordered.  Amorphous solids lack this order.  Can tell is a solid is crystalline by X-ray diffraction...