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Intermolecular Bonding

Intermolecular Bonding. Intermolecular Forces. -Forces that act between stable molecules and macromolecules -include momentary attractions between molecules, diatomic molecules, and individual atoms.  -Intermolecular forces are due to differences in charge density in molecules. .

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Intermolecular Bonding

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  1. Intermolecular Bonding

  2. Intermolecular Forces -Forces that act between stable molecules and macromolecules -include momentary attractions between molecules, diatomic molecules, and individual atoms.  -Intermolecular forces are due to differences in charge density in molecules.

  3.  London Dispersion Force -Van der Waals, or London dispersion forces are caused by temporary dipoles created when electrons are lopsided.  -The electrons are constantly orbiting the nucleus, and by chance they could end up close together. The uneven concentration of electrons could make one side of the atom more negatively-charged than the other, creating a temporary dipole.  As there are more electrons in an atom, and the shells are further away from the nucleus, these forces become stronger.

  4. London Dispersion Forces Van der Waals forces explain how nitrogen can be liquified. Nitrogen gas is diatomic; its equation is N2. Since both atoms have the same electronegativity, there is no dipole and the molecule is non-polar. Van der Waals forces allow otherwise non-polar molecules to have attractive forces. These are by far the weakest forces that hold molecules together.

  5. Dipoles -Covalent bonds can be polar or non-polar, and so can the overall compound depending on its shape.  -When a bond is polar, it creates a dipole, a pair of charges (one positive and one negative).  -If they are arranged in a symmetrical shape, so that they point in opposite directions, they will cancel each other. For example, since the four hydrogens in methane (CH4) are facing away from each other, there is no overall dipole and the molecule is non-polar. In ammonia (NH3), however, there is a negative dipole at the nitrogen, due to the asymmetry caused by the non-bonding electron pair. The polarity of a compound determines its intermolecular bonding abilities.

  6. Covalent Bonds -Silicon dioxide forms a covalent network.  -Silicon forms only single covalent bonds -As a result, the individual molecules covalently bond into a large network. -These bonds are very strong (being covalent) and there is no distinction between individual molecules and the overall network.  -Covalent networks hold diamonds together. Diamonds are made of nothing but carbon, and so is soot. Unlike soot, diamonds have covalent networks, making them very hard and crystalline.

  7. Hydrogen Bonds -Hydrogen bonds will occur when a hydrogen atom is attached to an oxygen, nitrogen, or fluorine atom.  -As a result of their high electronegativities, they pull the electrons almost completely away from the hydrogen.  -It becomes a bare proton sticking out from the molecule, and it will be strongly attracted to the negative side of any other polar molecules.  -Hydrogen bonding is basically an extreme type of dipole-dipole bonding. These forces are weaker than intramolecular bonds, but are much stronger than the other forces, causing these compounds to have high boiling points.

  8. Hydrogen Bond

  9. Dipole-Dipole Bonds -When two polar molecules are near each other, they will arrange themselves so that the negative and positive sides line up.  -There will be an attractive force holding the two molecules together, but it is not nearly as strong a force as the intramolecular bonds.  -This is how many types of molecules bond together to form large solids or liquids.

  10. Dipole-Dipole Bonds

  11. Polar and Nonpolar Shapes -When a molecule has a linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral shape, it will be non-polar. -These are the shapes that do not have non-bonding pairs. The other shapes (with non-bonding pairs) will be polar. Unless, of course, covalent bonds are non-polar, in which case there would be no dipoles to begin with.

  12. Melting and Boiling Points -To determine which substance has the higher melting or boiling point, you must decide which one has the strongest intermolecular force.  -Metallic bonds, ionic bonds, and covalent networks are very strong, as they are actually intramolecular forces. These substances have the highest melting and boiling points because they only separate into individual molecules when the powerful bonds have been broken.  -Breaking these intramolecular forces requires great amounts of heat energy. -Substances with hydrogen bonding will have much higher melting and boiling points than those that have ordinary dipole-dipole forces. Non-polar molecules have the lowest melting and boiling points, because they are held together by the weak van der Waals forces.

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