Molecular Shape, Electronegativity

Question 1

What are the different molecular shapes?

Answer 1

Linear 
Trigonal 
Tetrahedral 
Trigonal pyramidal 
Bent 
Trigonal Bipyramidal
Octahedral

Question 2

What are the features of a linear molecule?

Answer 2

2 atoms attached to the central atom = has 2 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 180

Question 3

What are the features of a trigonal molecule?

Answer 3

3 atoms attached to the central atom - has 3 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 120

Question 4

What are the features of a tetrahedral molecule?

Answer 4

4 atoms attached to the centra atom - has 4 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 109.5 = atom are as far apart as possible

Question 5

What are the features of a trigonal pyramidal molecule?

Answer 5

3 atoms attached to the central atom and 1 lone pair = 4 electrons domains
bond angle - 107
- lone pair exerts extra repulsion on the 3 bonding pairs
- lone pair contracts the bond angle
- decreases bond angle from 109.5 to 107

Question 6

What are the features of a bent molecule?

Answer 6

2 atoms attached to the central atom and 2 lone pair = 4 electrons domains
bond angle - 104.4
- lone pair exerts extra repulsion on the 2 bonding pairs
- lone pair contracts the bond angle
- decreases bond angle

Question 7

What are the features of a trigonal bipyramidal molecule?

Answer 7

5 atoms attached to the central atom - 5 electron domains
bond angle - 90, 120
2 axial positions and 3 equatorial positions

Question 8

What are the features of a octahedral molecule?

Answer 8

6 atoms attached to the central atom - 6 electron domains

bond angle - 90

Question 9

What is VSEPR theory?

Answer 9

Valence Shell Electron Pair Repulsion
- count the valence electrons (group number)
- complete the octet around the central atom = make double, triple bonds if there aren’t enough electrons to give an octet
- calculate formal charges
formal charges = valence electrons - electrons in the atom
- sum up the number of electron domains
- find the geometry and bond angle

Question 10

What are electrons domains?

Answer 10

each bond is an electrons domain

lone pairs count for one electrons domain

Question 11

What is electronegativity?

Answer 11

tendency of an atom to attract a bonding pair of electrons within a covalent bond

Question 12

How does electronegativity change across the periodic table?

Answer 12

increases from the left to right across a period
- has more electrons in the orbitals and more protons in the nucleus. this causes the nucleus to contract as attractive force increases

decreases down a group
- valence electrons are in shells further away from the nucleus so there is weaker attractive force from the nucleus

no trend in transition metals

Question 13

What are the three classifications of intramolecular bonding regarding electronegativity difference?

Answer 13

non-polar covalent

• bonds whom occur between atoms with little or no electronegativity difference
• less than 0.5

polar covalent

• bonds which occur between atoms with definite electronegativity difference
• between 0.5 and 2.0

ionic

• bonds which occur between atoms with large electronegativity difference where electron transfer can occur
• greater than 2.0

Question 14

What are intermolecular forces?

Answer 14

intermolecular forces are weaker than intramolecular forces

intra - forces within the compound/ molecules (holds molecule together)
inter - forces between molecules

intermolecular forces

• determines the physical properties of molecules = fusion, vaporisation
• boiling point can be used to reflect the strength of intermolecular forces = the higher the bp, the stronger the intermolecular forces

Question 15

Why don’t elements other than fluorine in group 7 form hydrogen bonds when in the hydrogen halide form?

Answer 15

electronegativity decreases down group 7

• fluorine is the most electronegative
• H-X bond for the other halides is less polar therefore cannot form hydrogen bonds
• lone pairs for the other halides are in higher energy levels. this makes the lone pairs bigger so they don’t have an intensely concentrated negative charge for hydrogens to be attracted to

Question 16

What is hydrogen bonding?

Answer 16

formed when there are functional groups with N, S or O present and there is a H atom linked to it

hydrogen bonds are directional
- H bonds are stronger when all the atoms are in a straight line

requires H atoms directly bonded to an electronegative atom and an electron acceptor (atom with a lone pair or negative charge) to occur

Question 17

Why does ice float?

Answer 17

liquid water molecules - bonds are continually forming and breaking

ice water molecules - forms 4 H bonds = ordered open arrangement
- O can make 2 bond and H can make 1 bond each

Question 18

What are London Dispersion Forces?

Answer 18

London dispersion forces are the weakest intermolecular forces in neutral molecules

• applies to covalent molecules with no dipole moment
• result of the motion of electrons

electrons distribution may become uneven at any point resulting in an instantaneous dipole. this can induce a dipole in neighbouring atoms. these atoms are then weakly attracted to each other
- temporary, small and local instantaneous dipole

Question 19

What is the trend in intermolecular forces of different alkanes?

Answer 19

the larger the alkane, the larger the induced temporary dipole-dipole force
- because they have more electrons and are more polarisable