BONDING

Diapositiva 1

• POLARISATION
• Electronegativity
• The ability to attract the bonding electrons in a covalent bond.
• Measured on the Pauling Scale.
• High number=Element is better able to attract the bonding electrons. F=most EN element. O,N,Cl=also V.Strongly En.
• Polar and non-polar bonds.
• The covalent bonds in diatomic gases (e.g. H2, Cl2) are non-polar because the atoms have equal EN's, and are TFR the electrons are equally attracted to both nuclei. Some elements like C and H, have pretty similar EN's, so bonds between them are essentially non-polar.
• In a covalent bond between 2 atoms of different EN's, the bonding electrons=pulled towards the more EN atom. This makes the bond polar. The greater the diff in EN, the more polar the bond.
• In a polar bond, the difference in EN between the 2 atoms causes a DIPOLE
• |__> A difference in charge between 2 atoms caused by a shift in Electron Density in the bond.
• Tip: The atoms in polar bonds generally have an EN difference that is greater than 0.4.

Diapositiva 2

• POLAR MOLECULES
• If charge is distributed unevenly over a whole molecule, then the molecule will have a PERMANENT DIPOLE.
• Mols that have a permanent dipole are called POLAR MOLS. Whether or not a mol is polar depends on whether it has any polar bonds, and its overall shape.
• In simple mols, e.g. HCl, The one polar bond means charge is distributed unevenly across the whole molecule, so it has a permanent dipole.
• The arrow means there is a permanent dipole, so the molecule is polar. It points from the positive to the negative end of the molecule.
• More complicated molecules might have several polar bonds. The shape of the molecule will decide whether or not it has an overall permanent dipole. If the polar bonds are arranged symmetrically so that the dipoles cancel each other out, such as in Carbon Dioxide, then the molecule has no permanent dipole and is non-polar.
• if the polar bonds are arranged so that they all point in roughly the same direction, and they don't cancel each other out, then charge will be arranged unevenly across the whole molecule. This results in a polar molecule-the molecule has a permanent dipole.
• Relationship between Electronegativity, polarisation and dipoles:
• Differences in the EN of atoms cause bonds to become polarised, which results in a dipole-a difference in charge between the 2 atoms.
• If these dipoles don't cancel out, then the molecule will be polar (it will have a permanent dipole.)

Diapositiva 3

• Intermolecule Forces.
• What are IM forces?
• Forces between mols. Much weaker than covalent ionic, metallic bonds.
• 3 types: [Weakest]van der Waals (induced dipole-dipole/London Forces/Dispersion Forces), permanent dipole-dipole, Hydrogen bonding (strongest type).
• Van der Waals forces [Temporary dipoles are also called induced dipoles.]
• Cause all atoms and mols to be attracted to each other.
• Electrons in charge clouds are always moving v. quickly.
• At any particular moment, the electrons in an atom are likely to be more to one side than the other.
• At this moment, the atom would have a temporary dipole.
• This dipole can cause another temporary dipole in the opposite direction on a neighboring atom.
• The two dipoles are then attracted to each other. The second dipole can cause yet another dipole in the 3rd atom.
• Because the electrons are constantly moving, the dipoles are being created and destroyed all the time.
• Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
• E.g. VdW forces are responsible for holding Iodine mols together in a lattice. iodine atoms are held together in pairs by strong covalent bonds to form mols of I2.
• But the mols are then held together in a molecular lattice arrangement by weak VdW forces.
• Molecular lattices are a type of crystal structure.

Diapositiva 4

• Not all VdW forces are the same strength--larger mols have larger electron clouds, meaning stronger VdW forces.
• The shape of mols also affects the strength of VdW forces:
• -Long, straight molecules can lie closer together than branched ones--the closer together 2 mols are, stronger the forces between them.
• When you boil a liquid, you need to overcome the IM forces, so that the particles can escape from the liquid surface. So, you need more energy to overcome stronger IM forces, so liquids with stronger VdW forces will have higher melting Boiling Points.
• VdW forces affect other physical properties, e.g. Melting point and viscosity.
• TIP: There are VdW forces between the mols in every chemical.
• E.g.1.):
• As you go down the group of noble gases, the number of electrons increases. So the VdW forces increase, and so do the boiling points.
• E.g.2.):
• As the alkane chains get longer, the number of electrons in the mols increases.
• The area over which the VdW forces can act also increases.
• This means the VdW forces are stronger, and so the Boiling Points increase.

Diapositiva 5

• Permanent Dipole-Dipole Forces.
• In a substance made up of mols that have permanent dipoles, there will be weak electrostatic forces of attraction between the delta positive and delta negative charges on neighbouring mols. These are called Permanent Dipole-Dipole Forces.
• E.g:
• Hydrogen Chloride gas has polar molecules due to the difference in EN of H and Cl.
• -The mols have weak electrostatic forces between them because of the shift in electron density.
• If you put an electrostatically charged rod next to a jet of a polar liquid, e.g. water, the liquid will move towards the rod.
• This is because polar liquids contain mols with permanent dipoles. It doesnt't matter if the rod is positively or negatively charged. The polar mols in the liquid can turn around so the oppositely charged end is attracted towards the rod.
• The more polar the liquid, the stronger the electrostatic attraction between the rod and the jet, so the greater the deflection will be.
• By contrast, liquids made up of non-polar molecules, e.g. Hexane, will not be affected at all when placed near a rod.

Diapositiva 6

• TIP: Hydrogen bonding is a special case scenario as it only happens in specific molecules.
• Not EVERY molecule with Hydrogen in it makes Hydrogen bonds.
• HYDROGEN BONDING.
• The strongest IM force. Only happens when H is covalently bonded to F, N, O.
• F, N, O = V. EN, so they draw the bonding electrons away from the H atom.
• [CHARGE DENSITY: A measure of how much positive or negative charge there is in a certain volume]
• The bond is so polarised, and Hydrogen has such a high charge density because it;s so small, that the Hydrogen atoms form weak bonds with the lone pairs of electrons on the F, N, O atoms of other mols.
• Mols which have H bonding are usually organic, containing -OH and -NH groups.
• Hydrogen bonding has a huge effect on the properties of substances.
• Susbtances w/H bonds have higher BP's and MP's than other similar mols because of the extra energy needed to break the H bonds.
• This is the case w/ H2O wh/ has a much higher BP than the other group 6 Hydrides.
• TIP: The melting points of compounds that form Hydrogen bonds are higher than other simple covalent compounds, but still much lower than ionic or giant covalent substances.
• The effects of H bonding=V.apparent in other physical properties of H2O too.
• As liquid H2O cools to form ice, the mols make more H bonds and arrange themselves into a regular lattice structure.
• Since H bonds are relatively long, the avg. Distance between H2O mols is greater in ice than in liquid water, so ice is less dense than liquid water.
• This is unusual as most substances are more dense as solids than they are as liquids.