Shapes of Molecules and Intermolecular Forces

• a solid line represents a bond in the plane of the paper
• a solid wedge represents a bond coming out of the plane of the paper
• a dotted wedge represents a bond going into the plane of the paper
• a lone pair is slightly closer to the central atom which occupies more space than a bonded pair. this results in a lone pair repelling more strongly than a bonding pair.
• electronegativity is the attraction of a bonded atom for the pair of electrons in a covalent bond.
• paulings electronegativity values depend on the elements position in the periodic table. across the periodic table:


  nuclear charge increases

  atomic radius decreases
• what are the most electronegative atoms?

  nitrogen, oxygen, fluorine and chlorine
• what are the least electronegative atoms?

  group one metals, including lithium, sodium and potassium.
• in a non polar bond, the bonded electron pair is shared equally between the bonded atoms.
• a bond is non polar when

  the bonded atoms are the same

  the bonded atoms have the same or similar electronegativity
• in molecules where the atoms come from the same element, the electron pair is shared equally and so is a pure covalent bond
• in a polar bond, the bonded electron pair is shared unequally between the bonded atoms. a bond is polar when the bonded atoms have different electronegativity values, resulting in a polar covalent bond.
• what is a dipole?

  a separation in opposite charges
• the atom with the larger electronegativity value has the $\delta-\$charge
• a dipole in a polar covalent bond does not change and is called a permanent dipole dipole
• london forces (induced dipole-dipole)


  exists between all molecules

  are only temporary
• the more electrons in each molecule:

  the larger the instantaneous and induced dipoles

  the greater the induced dipole-dipole interactions

  the stronger the attractive forces between molecules
• larger number of electrons mean larger induced dipoles. more energy is needed to overcome the intermolecular forces, increasing the boiling points
• in a simple molecular lattice, weak intermolecular forces can be broken easily at low temperatures.
• when a simple molecular structure is broken apart during melting, only the weak inter molecular forces break. covalent bonds are strong and do not break.
• non polar simple molecular substances tend to be soluble in non polar solvents
• why cant simple molecular structures conduct electricity?

  there are no moving charged particles that can move, so nothing can complete an electric circuit.
• hydrogen bond a special type of permanent dipole-dipole interaction found between molecules containing

  an electronegative atom with a lone pair of electrons (NOF)

  a hydrogen atom attached to an electronegative atom
• london forces

  when electrons in one atom move to one side spontaneously, causing neighbouring atoms electrons to repel, creating a dipole
• what are the strongest IMF's

  hydrogen bonds
• why is ice less dense than water?


  hydrogen bonds hold water molecules apart in an open lattice structure

  water molecules in ice are further apart
• permanent dipoles


  between a carbon and group seven element. will occasionally have the delta negative/positive signs
• sigma bond


  when a shared pair of electrons is directly shared along an internuclear axis
• pure covalent

  electronegativity difference <0.4
• polar covalent

  electronegativity difference between 0.4 and 1.8
• ionic bond

  electronegativity difference >1.8