6 Shapes of molecules and IMF

what is electron repulsion theory

• Pairs of electrons repel each other so will arrange themselves as far away as possible from each other

• electron pairs surrounding a central atom therefore determine the shape of a molecule or ion

• different numbers of electrons around an atom result in different shapes

amount of repulsion in pairs from greatest to least

1. Lone pair - lone pair (greatest)

2. lone pair -bonded pair

3. bond pair - bond pair (least)

why do lone pairs repel the most

lone pairs repel the most because they are slightly closer to the central atom therefore occupy more space

how much does a lone pair decrease the bond angle

2.5 degrees

if you have a multiple bond (e.g. double) does this count as more than one region of electrons

no

what is a molecule with 6 bonded pairs known as

octahedral

Name the shapes you need to know

• linear

• trigonal planar

• tetrahedral

• pyramidal

• bent / non-linear

• trigonal bipyramid

• octahedral

why is methane tetrahedral

4 bonding regions

all bonding regions are bonded pairs

repulsion is equal

tetrahedral

why is ammonia pyramidal

4 bonding regions

1 lone pair

lone pair repels bond pairs more than bond pairs

pyramidal

why is water non-linear/bent

4 bonding regions

2 lone pairs that repel the most

followed by lone pair and bonded pairs which repel

least repulsion is between bond pair - bond pair

non linear/bent structure

What is a covalent bond

strong electrostatic bond between a shared pair of valence electrons and the nuclei of the bonded atoms

however electrons in a covalent bond aren’t always shared equally, some atoms are more electrically attractive than others

what is electronegativity and the name of the scale that measures it

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

the Pauling scale

what is the link between bond type and electronegativity difference

covalent bonds have an electronegativity difference of 0

polar covalent bonds (e.g. HCl) have an electronegativity difference of 0 to 1.8

ionic bonds have an electronegativity difference greater than 1.8

• this is why Beryllium Iodide is a covalent molecule despite having a metal and a non metal

factors than affect electronegativity

1. size of nuclear pull/charge (increases across a period as number of positive protons increases)

2. distance of electrons to pull of nucleus/atomic radius (decreases up group)

nonpolar covalent bond

Bonded electron pair is shared equally between bonded atom because the bonded atoms are the same or have similar/the same electronegativity

e.g. chlorine

all diatomic molecules are nonpolar (very limited/no electronegativity difference)

do nonpolar solvents mix with water

no - like dissolves like and water is polar

polar covalent bond

bonded electron pair is not shared equally between bonded atoms because they have different electronegativity

e.g. HCl

what is the polarity of a bond that has a permanent dipole

polar covalent bond is a permanent dipole

when is there an overall dipole

any shape NOT

trigonal planar or tetrahedral

why is water able to dissolve NaCl

water molecules are able to break down the ionic lattice due to the partially negative oxygen ions and the partially positive hydrogen ion.

1. water molecules attract Na+ and Cl- ion

2. the ionic lattice breaks down as it dissolves

3. in the resulting solution, water molecules surround the Na+ and Cl- ions

Na+ ions are attracted to the δ- oxygen of the water molecules

Cl- ions are attracted to the δ+ hydrogen of the water molecules

Permanent dipole dipole interaction

Name the 3 types of IMF

• induced dipole dipole interactions (london forces)

• permanent dipole dipole interactions

• hydrogen bonding

what are IMF responsible for and what do covalent bonds determine

IMF are responsible for physical properties (mpt and bpt)

covalent bonds determine the identity and chemical reactions of molecules

what is the bond enthalpy for London forces

1-10

bond enthalpy for permanent dipole dipole interactions

3-25

bond enthalpy for hydrogen bonds

10-40

bond enthalpy for single covalent bonds

150-500

def London forces

London forces are weak IMF that exist between ALL molecules and atoms, whether polar or non-polar. They act between induced dipoles in different molecules.

how do London forces occur

1. movement of electron cloud 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 neighbouring molecules which attract one another

the MORE electrons in each molecule…

• the larger the instantaneous and induced dipoles

• the greater the induced dipole-dipole interactions

• the stronger the attractive forces between molecules

• more energy needed to overcome the IMF and separate molecules

• higher bpt

larger numbers of electrons mean larger induced dipoles therefore greater induced dipole-dipole interactions therefore stronger attractive forces between molecules therefore more energy is needed to overcome the IMF and separate molecules therefore higher bpt

what molecules form permanent dipole interactions

polar molecules

the δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules.

these happen in ADDITION to (not instead of) London forces

def hydrogen bonds

a special type of permanent dipole interactions found between molecules containing

• an electronegative atom with a lone pair of electrons (Oxygen, nitrogen, fluorine)

• a hydrogen atom covalently bonded to an electronegative atom

why is solid ice less dense than water

• hydrogen bonds hold water molecules in an open tetrahedral lattice structure with holes in it. this open structure decreases the density of water on freezing

• when ice melts the ice lattice collapses and molecules move closer together, therefore the water molecules in ice are further apart than in water therefore less dense

why does water have a higher mpt and bpt than other simple molecules of similar size and mass?

each water molecules can form 4 hydrogen bonds

• 2 lone pairs on the oxygen

• 2 hydrogen atoms

this leads to a higher mpt and bpt as there are more IMF that need to be overcome for the molecules to be separated therefore more energy is needed

list of shapes u need to know

• linear

• trigonal planar

• tetrahedral

• pyramidal

• bent/non-linear

• trigonal bipyramid

• octahedral

bond angle for linear

180

bond angle for trigonal planar

120

bond angle for tetrahedral

109.5

bond angle for pyramidal

107

bond angle for bent/non-linear

104.5

bond angles for trigonal bipyramid

120 and 90

bond angle for octahedral

90

why is octahedral called octahedral

if the points are joined up it makes an octahedron

trigonal bipyramid diagram

example for linear molecule

CO₂

example for trigonal planar

 BF₃

example for tetrahedral

CH₄

example for pyramidal

ammonia NH₃

example for bent/non-linear

water H₂O

example for trigonal bipyramid

PF₅

example for octahedral

SF₆

funny molecules

S₈

P₄

most electronegative atom on pauling scale

fluorine

electronegativity difference for covalent bonds

0

electronegativity difference of polar covalent bonds

0-1.8

electronegativity difference for ionic bonds

greater than 1.8

what does an overall dipole look like

what do permanent dipoles look like

example of a polar molecule

water

when is there no overall dipole

when the dipoles cancel out

how to tell if a molecule is polar or non-polar

• does it have symmetry?

  • tetrahedral shapes r only non-polar if the atoms on the ends are all the same

• do the dipoles cancel out?

  • C and H are close enough in electronegativity that they cancel out