3.1.3.6 Bond polarity

• What is electronegativity?


  The power of an atom to attract a pair of electrons in a covalent bond.
• What is the electronegativity in a covalent bond when electron distribution is not the same?


  Unsymmetrical
• What is electronegativity measured on?


  The pauling scale 0-4
• What is the most electronegative element?


  Fluorine
• What are the most electronegative atoms?


  F, O, N, Cl
• Factors that affect electronegativity


  Number of protons

  Atomic radius

  Shielding
• Why does electronegativity increase across a period?


  • Number of protons increases so stronger attraction between nucleus and bonding pair of electrons
  • Atomic radius decreases because the electrons in the same shell are pulled in more.
  • Shielding is similar
• Why does electronegativity decrease down a group?

  • Distance increases so attraction between nucleus and outer electrons decreases
  • Shielding increases
  • Atomic radius increases
• Which element in a molecules will be polar?

  The element with the larger electronegativity
• What will a compound which has a small electronegativity difference be?


  Purely covalent
• What will a compound containing a large difference in electronegativity be?


  Ionic
• How does a permanent dipole form in a polar bond? 


  A polar covalent bond forms when the elements have different electronegativities. This means it has an unequal/ unsymmetrical distribution of electrons/ charge density so produces a charge separation (dipole) s+ and s- ends
• What is a symmetrical molecule?


  A symmetric molecule (all bonds identical and no lone pairs) will not be polar even if individual bonds within the molecular are polar. The individual dipoles on the bonds ‘cancel out’ due to the symmetrical shape of the molecule. There is no net dipole moment: the molecule is non-polar
• What do atoms in a polar molecule usually have an electronegativity difference of?


  0.4
• What are non polar bonds?


  Atoms have equal electronegativities so both electrons are equally attracted to the nuclei. This results in a symmetrical distribution of electron density around the two atoms.
• What is a dipole?


  In a polar bond, the difference in electronegativity between the two atoms causes a dipole which is a difference in charge between two atoms caused by a shift in electron density in the bond
• What is a polar molecule?


  If charge is distributed unevenly over a molecules then the molecule will have a permanent dipole. This depends on the shape e.g. HCl = charge distributed unevenly so dipole.
• How does nuclear charge affect electronegativity?


  More protons, stronger attraction between the nucleus and bonding pair of electrons 
• How does atomic radius affect nuclear charge?


  As radius of an atom increases, the bonding pair of electrons become further from the nucleus. They are therefore less attracted to the positive charge of the nucleus, resulting in a lowerelectronegativity.
• How does shielding affect nuclear charge?


  Less shells of electrons between the nucleus and the electrons, less shielding (less repulsion), stronger attraction between the nucleus and bonding pair of electrons 
• Are less polar molecules ionic or covalent?


  Covalent
• Are more polar molecules ionic or covalent?


  Ionic as the more electronegative atom attracts the electrons in the bond enough to ionize the other atom.
• How does the arrangement of a molecule decide whether it is polar or non-polar?


  • If the polar bonds are arranged  symmetrically, the partial charges cancel out and the molecule is non-polar. 
  • If the polar bonds are arranged asymmetrically, the partial charges do not cancel out and the molecule is polar.