BONDING 2 - Electronegativity, Bond Polarity, and IMF Forces

• What is electronegativity?


  Ability of an atom to attract bonding electrons in a covalent bond
• Does electronegativity increase or decrease

  across a period?INCREASESin electronegativityACROSSa period
• Does electronegativity increase or decrease

  down a group?DECREASES DOWN a group

  INCREASES UP a group
• What does trend in electronegativity depend on?

  Its position in periodic table
• What are the bonding electrons in a non polar covalent bond like?

  Evenly distributed between both atoms - there is no dipole because there is no charge separation
• Which is the most electronegative element?

  Fluorine
• What is the Pauling Electronegativity

  Scale to interpret electronegativity values in molecules
• What is a Polar Bond


  Bond between atoms with different electronegativities, creating a permanent dipole
• can a covalent bond be polar?

  yes - depending on how unevenly distributed atoms are in covalent bond
• Why and how are polar bonds formed?

  - Bonded atoms have diff. electronegativities

  - atom w. greater electronegativity will attract the bonding

  electrons more strongly than the other atom.

  - The e- in polar bond are therefore unevenly distributed

  - This leads to a small charge separation within the covalent bond.

  - This small charge separation is called a permanent dipole

  - The more electronegative atom has a small negative charge (δ-)

  The less electronegative atom has a small positive charge (δ+)

  
• what is a Permanent Dipole


  Small charge separation within a covalent bond due to uneven electron distribution
• Polar Molecule


  Molecule with an overall dipole due to permanent dipoles and molecular shape
• Is H-Cl polar or non polar and why?

  - Electron pair attracted towards the more electronegative Cl atom

  - There's an Uneven distribution of electrons

  between atoms

  - Therefore has a polar covalent bond
• Is Cl-Cl polar or non polar and why?

  - It is Polar

  - This is because both atoms have an equal electronegativity

  - therefore electrons are equally distributed between atoms
• Is it possible for a molecule containing polar bonds to be non-polar? Explain why?

  YES- It's important to consider whether molecule is symmetrical or non-symmetrical.-If molecule issymmetrical,dipolesof any bonds willcancel+ the moleculewill be non-polar.See an expert-written answer!We have an expert-written solution to this problem!
• Is CO2 polar or non polar? Why?

  CO2 molecule is non-polar- even though eachC=O bond is polar.- BECAUSE CO2 molecule issymmetrical- dipoles cancel out
• Is CCl4 polar or non polar? Why?

  CO2 molecule is non-polar- Even though C-Cl bond is polar- The C-Cl4 molecule is symmetrical- Therefore dipoles cancel out making it non polar
• When will a molecule be polar?

  A molecule will be polar if it containspolar bondsandhas anon-symmetricalshape.
• Why does a symmetrical shape make a molecule, even with polar bonds, non-polar

  All the dipoles cancel each other out making it non-polar
• EXAM STYLE Q FORMAT: When asked why one molecule is nonpolar and another is polar.. what should you mention? (3)

  - Theshapeof non polar molecule and the fact is issymmetrical

  -Because it is symmetrical, the dipoles cancel each other out, making it non-polar

  -Shapeof polar molecule and the fact it isNOT SYMMETRICAL
• Intermolecular Forces


  forces of attraction that occurbetweensimple covalent molecules.

  
• 3 types of intermolecular forces

  - London forces (induced dipole-dipole)

  - Permanent dipole dipole

  - Hydrogen bonds
• Order from strongest to weakest intermolecular forces?

  - STRONGEST = Hydrogen bonding

  - Permanent dipole-dipole

  - WEAKEST = London's forces
• London Forces (Induced dipole - dipole attraction) DEFINITION (3MARKS)


  -Uneven distribution of electrons causes a temporary dipole on one molecule

  - This induces an opposite dipole onto the next molecule
• When can London's forces and temporary dipole only exist?

  When two molecules / atoms are near by -> When they move away - dipole interaction destroyed
• What kind of structures can induced dipole-diploe forces hold?

  Crystal structures such as Iodine, which hold I2 molecules together
• When boiling a liquid do you break London's forces or covalent bonds?

  London's forces
• Why does boiling point of straight hydrocarbon chains increase as they get longer?

  - They have a longer chain length AND therefore more surface contact therefore

  -stronger and more induced dipole-dipole forces
• Why do branched molecules / hydrocarbons have a lower boiling point?

  - Less surface contact which means molecules can't be packed together as closely, which weakens induced dipole-dipole forces
• What two factors of a chain of a molecule / hydrocarbon affects it's boiling point and why?

  - If it'schain longer- And it isnot branchedwhich causesgreater surface contact- Has stronger London forces,whichrequire more energy to break
• Can London's forces exist between all molecules?

  Exist between all molecules, but most obvious in non-polar molecules
• Permanent dipole dipole DEFINITION

  -permanent dipolein one molecule will attract theopposite permanent dipolein a neighbouring molecule resulting in apermanent dipole – dipole force.
• Can Permanent dipole dipole exist between all molecules?

  ONLY exist betweenpolarmolecules but can also have London's forces too
• Why is London's forces weaker than permanent dipole dipole?

  Unlike London's forces, dipole interactions involve molecules w. a permanent dipole, so are stronger
• What must be present for hydrogen bonding to occur?

  To have hydrogen bonding:- aO-H, N-H or F-H bondmust be present- ANDlone pairon aneighbouring moleculeon theO or Nmust be available
• Hydrogen Bonding DEFINITION


  - The attraction between anelectron deficient H delta +on one molecule- and alone pair of electrons on anF: O: or N:on a different molecule (as these elements are very electronegative)
• Does hydrogen bonding occur in all molecules that have O-H or N-H?

  hydrogen bonding does not occur in all molecules that have hydrogen, the above O-H or N-H is essential
• EXAM Q STRUCTURE: When asked to draw hydrogen bonding, what checklist should you make? (6 points)

  Diagram check list:- 2 separate molecules shown

  - Lone pairs on both molecules shown

  - Correctly labelled negative and positive Dipoles included

  - Hydrogen bond drawn correctly with patchy line

  - HYDROGEN BOND LABELLLED AS A HYROGEN BOND

  - Hydrogen bond line drawn from the centre of lone pair to next hydrogen of other atom
• 4. Draw a diagram to illustrate hydrogen bonding between 2 molecules of CH3OH

  
• flow diagram to give you an easy way to predict the type of intermolecular forces present in any given molecule.