chapter 6

• the electron pairs surrounding the central atom determine the shape of the molecule
• electron pairs repel each other as far as possible in order to hold a definite shape
• a lone pair is slightly closer to the central atom and occupies more space, resulting in it repelling more strongly than a bonded pair
• shapes with no lone pairs and only bonding atoms:

  2= linear 180

  3= triagonal planar 120

  4=tetrahedral 109.5

  6=octahedral 90
• shape with lone pairs and bonding pairs=

  3 bonded pairs and 1 lone= pyramidal 107

  2 bonded 2 lone= non linear 104.5
• electronegativity is the attraction of a bonded atom for the pair of electrons in a covalent bond
• the pauling scale is used to compare electronegativities of atom on the periodic table. electronegativity increases as you go up the periods and across to the right of the groups
• bond types due to electronegativity difference-

  covalent = 0

  polar= 0-1.8

  ionic= 1.8+
• in a non polar bond, the bonded electron pair is shared equally. the bonded atoms are usually the same, or have the same/ similar electronegativity, such as a c-h bond
• in a polar bond, the bonded electron pair is shared unequally between the bonded atoms due to different electronegativity values.
• example of a polar bond: H-Cl

  cl is more electronegative than h, so it attracts the bonded pair closer. this causes it to have a slightly negative charge on the cl and a slightly positive charge on the hydrogen. This separation of charge is called a permanent dipole
• a molecule can have polar bonds, but the molecule may not be polar.

  a water molecule is polar as the dipoles act in the same overall direction and do not cancel each other, whereas a carbon dioxide molecule has two dipoles acting in two overall different directions, so the dipoles cancel so it isn't polar.
• intermolecular forces are weak interactions between dipoles of different molecules. they fall into three main categories:

  -induced dipole interactions(London Forces)

  -permanent dipole interactions

  -hydrogen bonding
• London forces are weak intermolecular forces that exist between all molecules. they originate from induced dipoles:

  -movement of electrons produce a changing dipole in the molecule causing a instantaneous dipole at any instant that is constantly shifting.

  -this dipole is induced on the neighbouring molecules, until they all attract one another
• the strength of london forces increase when there are more electrons in the molecules, as there are more induced dipoles so stronger attractive forces.
• permanent dipole interactions are stronger than London forces as they need extra energy to overcome more forces, causing a higher mp and bp
• a simple molecular substance is made of simple molecules. they form a regular structure called a simple molecular lattice where the molecules are held together by weak intermolecular forces, but the molecules are full of atoms with strong covalent bonds.
• simple molecular substances have a low mp and bp due to weak intermolecular forces that don't require a lot of energy to overcome.
• simple covalent substances are non conductors of electricity as there are no mobile charged particles.
• non polar simple molecular substances only dissolve in non polar substances as intermolecular forces form between the molecules and solvent. when they're added to a polar substance, there is little interaction between the molecules are solvent as the intermolecular bonding in the polar solvent is too strong to be broken
• polar simple molecular substances may dissolve in polar substances as the molecules in the substance and solvent attract each other . the solubility depends on the strength of the dipole and can be hard to predict
• a hydrogen bond is a special type of permanent dipole interaction found between molecules containing:

  -an electronegative atom with a lone pair ( o, f or n)

  -a hydrogen atom attached to an electronegative atom
• the hydrogen bond acts between a lone pair of electron and a hydrogen atom. they are the strongest intermolecular attraction.
• hydrogen bonding gives water some unique and anomalous properties:

  -ice is less dense than water ,as hydrogen bonds hold water molecules in ice further apart in an open lattice structure, so it floats

  -water has a high mp and bp, as lots of energy is required to overcome the strong hydrogen bonding in water