molecules

• a molecule is a particle made of a specificed number of atoms that are covalently bonded together
• a covalent bond is an electrostatic attraction between a shared pair of electrons situated between 2 non-metal atoms and the nuclei of the adjacent atoms
• a dative covalent bond is a covalent bond where both electrons that are being shared originate from the same atom
• a lone pair is two electrons in the same orbital which are currently not involved in forming covalent bonds
• electronegativity is a measure of how strongly a pair of electrons in a covalent bond is attracted to an atomic nucleus
• fluorine is the most electronegative element
• a molecule being polar means there is an uneven distribution of electrons. the region with the highest concentration of electrons becomes slightly negative and the region with the lowest concentration of electrons becomes slightly positive
• a dipole is a separation of opposite electrical charges.
• a polar molecule will always have a permanent dipole
• a permanent dipole-dipole attraction is a type of intermolecular force that exists between polar molecules
• lone pairs will push bonding pairs closer together decreasing the angle between them by 2.5 degrees per lone pair
• lone pairs create the strongest repulsion force then double and triple bonds are next in strength and single bonds create the weakest repulsion force
• molecules that have 2 bonding pairs and 0 lone pairs have a linear shape
• linear shape molecules have a 180 degree bond angle
• molecules that have 2 bonding pairs and 1 lone pair have a non-linear shape
• non-linear shaped molecules have a bond angle of 117.5 degrees
• molecules that have 2 bonding pairs and 2 lone pairs have a non-linear shape
• non-linear shaped molecules (2 lone pair) have a bond angle of 104.5 degrees
• molecules that have 3 bonding pairs and 0 lone pairs have a trigonal planar shape
• trigonal planar shaped molecules have a bond angle of 120 degrees
• molecules that have 3 bonding pairs and 1 lone pair have a pyramidal shape
• pyramidal shaped molecules have a bond angle of 107 degrees
• molecules that have 4 bonding pairs and 0 lone pairs have a tetrahedral shape
• tetrahedral shaped molecules have a bond angle of 109.5 degrees
• molecules that have 4 bonding pairs and 2 lone pairs have a square planar shape
• square planar shaped molecules have a bond angle of 90 degrees
• molecules that have 5 bonding pairs and 0 lone pairs have a trigonal bipyramidal shape
• trigonal bipyramidal shaped molecules have bond angles of 90 degrees and 120 degrees
• molecules that have 6 bonding pairs and 0 lone pairs have an octahedral shape
• octahedral shaped molecules have a bond angle of 90 degrees
• only molecules of identical elements have evenly distributed electrons within the covalent bond
• factors that affect attraction of electrons:

  • nuclear charge
  • atomic radius
  • electron shielding
• a dipole shows the direction of bond polarisation by an arrow
• the degree of ionic/ covalent character depends on the difference in the electronegativity values for 2 bonded atoms
• if the difference in electronegativity is more than 1.7 the molecule is primarily ionic
• if the difference in electronegativity is between 1.7 and 0.3 then the molecule is primarily polar covalent
• if the difference in electronegativity is less than 0.3 the molecule is non-polar covalent
• requirements for polar molecules:

  • polar bonds
  • asymmetric molecule
• in an asymmetric molecule the dipoles don’t cancel each other out which creates a permanent dipole
• a sigma bond is an atomic orbital overlap in the line of the nuclei