Shapes and IMFS

Van der Waals forces / Induced dipole-dipole

Weakest type of IMF

Act as an induced dipole between molecules

Substances w/ simple molecular structure consist of covalently bonded molecules held together with weak Vdw forces

Strength: depends on Mr of molecule and its shape (larger Mr - stronger IMFs) (straight chain > branched chain as they can pack closer together: reduces distance which force acts making it stronger)

Act between alkane chains
Effected by chain length and presence of branching

Permanent Diole

Type of IMF which acts between molecules with a POLAR BOND

The positive and negative dipoles on adjacent molecules attract each other and hold molecules together in a lattice-like structure

Stronger than vdW forces - more energy needed to overcome them, so higher mpts and bpts

Hydrogen Bonding

Strongest type of IMFs

Act only between hydrogen and 3 most EN atoms: N, F and O

Lone pair of e- on these atoms forms bond with positive dipole on hydrogen atom

Much higher mpts and bpts than atoms without H bonding

Type of IMF heavily influences physical properties

Water’s unusual properties

Simple molecule with unusually high mpts and bpts for size of molecule due to Hydrogen bonds

Hydrogen bonds also result in ice having much lower density than liquid water, as they hold the molecules in a rigid structure w/ lots of air gaps

Properties of alcohols due to Hydrogen bonding

Much higher bpts than alkanes w/ similar Mr value: one pair of electrons on Oxygen atom in alcohol is able to form hydrogen bonds witha hydrogen bonded to oxygen on a neighbouring alcohol molecule

Alcohols and water good solvents for compounds that can form H bonds in solution

Poor for dissolving of some polar molecules (halogenoalkanes) which cannot form H bonds

H- bonds in DNA

AT base pair held together by two H-bonds

GC base pair held together by three H-bonds

Determining molecule shapes

1. Find no. of e- pairs

2. Determine how many pairs are bonding / lone

3. Bonding pairs indicate basic shape, lone pairs indicate additional repulsion

Lone pair repulsion

Bond angle reduced by 2.5º

Linear

2 bonding e- pairs

0 lone pairs

180º bond angle

Non-Linear

2 Bonding pairs

2 lone pairs

104.5º angle

Trigonal Planar

3 bonding pairs

0 lone pairs

120º bond angle

Triangular Pyramid

3 bonding pairs

1 lone pair

107º bond angle

Tetrahedral

4 bonding pairs

0 lone pairs

109.5º bond angle

Trigonal Bipyramid

5 bonding pairs

0 lone pairs

180º and 120º bond angle

Octahedral

6 bonding pairs

0 lone pairs

90º bond angle

Electronegativity

Ability of an atom to attract the bonding electrons in a covalent bond towards itself

Depends on size and nuclear charge

Increases along period (AR decreases and charge density increases)

Decreases down group (shielding increases and AR increases → charge density decreases)

Ionic Character

Covalent bond

Polar covalent bond

Ionic bond

Can be permanent or induced

Permanent Dipole

Two bonded atoms w/ different electronegativities

More EN atom draws more negative charge towards itself and away from other atom
Polar molecule requires polar bonds that do not cancel due to direction

CO2 - contains polar bonds but symmetrical SO non-polar

Induced Dipole

Can form when eletron orbitals around a molecule are influenced by distributions of electrons on another particle