2.5 - Polar Bonds & Polar Molecules

• electronegativity


  the ability of an atom to attract a bonding pair of electrons
• trend in electronegativity across a period

  increases across a period because nuclear charge increases but number of Shielding electrons doesn't so attraction for shared pair of electrons increases
• trend in electronegativity down a group

  decreases down a group because number of shielding electrons increases & atomic radius increases so bonding electron is further away from nucleus so it's harder to attract bonding electrons
• Pauling Scale


  used to predict type of bonding between atoms based on difference in electronegativity
• covalent bond


  electrons are shared equally between atoms because they have very similar or identical electronegativities
• small electronegativity difference


  leads to polar covalent bond because one atom is more electronegative so electrons are more attracted to it
• polar covalent band


  shared electrons are drawn towards the most electronegative atom

  bonds have a positive pole and a negative pole
• large electronegativity difference


  leads to an ionic bond
• polar molecules


  a covalent bond in which there's a separation of charge between one atom and the other

  one is slightly positive, the other slightly negative due to different electronegativities
• debye units (D)


  tells you how polar a molecule is

  the higher the number the more polar the molecule
• dipole


  difference in charge between 2 atoms of a covalent bond caused by shift in electron density due to electronegativity difference

  polar molecules exist as dipoles - permanent charge
• polarity of diatomic molecules

  have the same electronegativity so are non-polar
• what are non-polar polyatomic molecules like

  molecule has several polar hands that are arranged so they point in opposite directions so cancel each other out

  electron density is evenly spread around molecule despite electron deficiency in the middle where it's partially positive
• non-polar molecules


  linear

  trigonal planar

  tetrahedral
• what are polar polyatomic molecules like

  polar bonds all roughly point in the same direction

  opposite ends of the molecule have opposite charges so electron density moves up or down
• polar polyatomic molecules


  trigonal pyramidal

  pyramidal
• polarising ability


  positive ions can polarise nearby negative ions
• factors affecting polarising ability

  site of positive ion: the smaller the greater the polarising ability

  charge of positive ion: the greater it is the greater the polarising ability

  size of negative ion: the bigger it is the easier to polarise
• what happens to dipoles when they are in an electrical field

  electrostatic forces live up dipoles

  random movements due to kinetic energy of molecules tend to disrupt alignment
• dipole moment 


  measure if overall polarity of a molecule

  vector quantity

  magnitude of charge x distance between charges

  units: debye units
• what does a bigger dipole mean in an electrical field

  bigger dipole moment (twisting effect)  
• polar liquids


  thin stream is attracted towards object with an electrostatic charge because polar molecules tend to move & rotate due to charge on one side of the molecule being attracted to opposite charge on the object