Chapter 6: Shapes of molecules and ions, electronegativity, polarity, intermolecular forces, hydrogen bonding

what is electron-pair repulsion theory?

electron pairs repel so that they are as far apart as possible → the arrangement of electrons minimises repulsion and therefore holds the bonded atoms in a definite shape

lines, wedges, and dotted wedges:

order of repulsion between bonded and lone pairs:

for every lone pair, a bond angle is reduced by…

2.5 degrees

4 bonded pairs + 0 lone pairs =

3 bonded pairs + 1 lone pair =

3 bonded pairs + 2 lone pairs =

in electron-repulsion theory, multiple bonds are treated as…

the same as single bonds

what shape does carbon dioxide have?

linear

bond angles + names for 2,3,4, and 6 electron pairs + no lone pairs:

SF₆ electron repulsion theory:

eight sides to the shape it makes

shape of ammonium ion:

• tetrahedral

• 4 bonded regios

• no lone pairs

shapes of carbonate, nitrate, and sulphate ions:

electronegativity def.

the attraction of a bonded atom for the pair of electrons in a covalent bond

general patterns of electronegativity on the periodic table:

a large Pauling value indicates that the atoms of an elements are…

very electronegative

e.g. Fluorine is the most electronegative atom, and has a Pauling value of 4.0

how can we use the difference in electronegativities between 2 atoms to tell what kind of bond it will be?

0 difference = covalent

0 - 1.8 = polar covalent

greater than 1.8 difference = ionic

in a non-polar bond, the bonded electron will be…

shared equally between atoms

a bond will be non-polar when:

• the bonded atoms are the same - e.g. F-F

• the bonded atoms have the same/similar electronegativities - e.g. H-C

when is a bond polar?

the bonded atoms are different, e.g. H-Cl, and have different electronegativity values

→ e.g. Chlorine is most electronegative, and so the bonded electron pair is attracted closer towards the Cl atom .

what does this mean in bond polarity → δ

small → this indicates a partial charge in a polar bond

def. of a dipole

the separation of opposite charges

if an atom has a larger electronegativity, which δ will it have?

δ- → the smaller will have a δ+

why is a water molecule polar?

2. the 2 O-H bonds are permanent dipoles

3. the 2 dipoles act in different directions (but not directly opposite!)

overall, the oxygen end of the molecules has a δ- charge, and the hydrogen has a δ+ charge.

why is a CO₂ molecule non-polar?

1. both C=O bonds are polar, and have permanent dipoles

2. the 2 dipoles act in opposite directions, exactly opposing each other

3. the whole molecule + dipoles cancel out → overall dipole = 0

polar solvents + polar solutes:

• positive ions are attracted to the δ- of the water

• negative ions are attracted to the δ+ of the water

compare the strengths of London forces, permanent dipole-dipole interactions, Hydrogen bonds, and single covalent bonds:

what are London forces?

the forces that act between instantaneous dipoles:

1. movement of electrons produces a changing dipole in a molecule

2. at any instant, an instantaneous dipole will exist, but its position is constantly shifting.

3. the instantaneous dipole induces a dipole on a neighbouring molecule

4. the induced dipole induces further dipoles on the neighbouring molecules, which then attract one another.

how does the strength of London forces vary between different elements/molecules?

• more electrons = larger instantaneous + induce dipoles

• greater the induced dipole-dipole interactions

• stronger attractive forces between molecules

this affects boiling point !!

permanent dipole-dipole interactions:

F-F and H-Cl have the same number of electrons + the same shape. The strength of their London forces should be very similar. Why is the boiling point of H-Cl so much higher than F-F?

The permanent dipole-dipole interactions

• H-Cl = polar molecule

• extra energy is needed to break these additional pd pd bonds

what do substances with pd pd forces also have?

london forces!!

what is a simple molecular substance?

made up of simple molecules → small units containing a definite number of atoms with a definite molecular formula → e.g. H₂O

forces in a simple molecular lattice:

• the molecules are held in place by weak intermolecular forces

• the atoms within each molecules are bonded together strongly by covalent bonds.

boiling points of simple molecular substances:

low melting + boiling point:

• only the weak intermolecular forces break

• the covalent bonds are strong and do NOT break

solubility of non polar and polar substances:

non-polar = soluble in non-polar solvents

polar = soluble in polar solvents

solubility of non-polar simple molecular substances

insoluble in polar solvents → i. bonding in polar solvent is too strong to be broken

soluble in non-polar solvents → intermolecular forces form between the molecules and the solvent → interactions weaken the intermolecular forces in the simple molecular lattice → i.forces break + compound dissolves

some compounds, like ethanol, that contain non-polar and polar parts can…

dissolve in non-polar and polar solvents

hydrophilic part (e.g. O-H) interacts with water

hydrophobic part (carbon chain) will not

can simple molecular structures conduct electricity?

no → no charged particles that can move

which 4 elements can be in a hydrogen bond?

F, H, O, N

hydrogen bond occurs between…

a lone pair + a hydrogen atom

draw a hydrogen bond between 2 water molecules:

why is ice less dense than water?

• Hydrogen bonds hold water molecules apart in an open lattice structure

• The water molecules in ice are held further apart than in water.

• Solid ice is less dense than liquid water and floats.

why does water have an unusually high melting point?

the hydrogen bonds!!

other unusual properties of water:

• relatively high surface tension

• high viscosity