2.5 shapes of simple molecules and ions

Question 1

why do molecules have a specific shape with specific angles?

Answer 1

• bonds repel each other equally
• bonds contain electrons so they will want to repel as far as possible

Question 2

lone pair + bond pair
vs
bond pair + bond pair

Answer 2

a lone pair next to a bond pair will repel more than 2 bond pairs together because lone pairs repel more
therefore, 2 lone pairs together repel even further

Question 3

why do lone pairs change the shape and bond angles

Answer 3

lone pairs push bonding pairs closer together

Question 4

linear (180°)

Answer 4

bond pairs : 2
lone pairs: 0

e.g. BeCl2

Question 5

trigonal planar (120°)

Answer 5

bond pairs: 3
lone pairs: 0

e.g. BF3

Question 6

tetrahedral (109.5°)

Answer 6

bond pairs: 4
lone pairs: 0

e.g. CH4

Question 7

trigonal bipyramidal (90°, 120°)

Answer 7

bond pairs: 5
lone pairs: 0

e.g. PCl5

Question 8

octahedral (90°)

Answer 8

bond pairs: 6
lone pairs: 0

e.g. SF6

Question 9

trigonal pyramidal (107°)

Answer 9

bond pairs: 3
lone pairs: 1

e.g. NH3

Question 10

bent/non-linear (104.5°)

Answer 10

bond pairs: 2
lone pairs: 2

e.g. H2O

Question 11

trigonal planar (120°)

Answer 11

bond pairs: 3
lone pairs: 2

e.g. ClF3

Question 12

electronegativity

Answer 12

the ability for an atom to attract electrons towards itself in a covalent bond
F is the most electronegative element

Question 13

link between electronegativity and ionic bonds

Answer 13

the bigger difference in electronegativity, the more ionic a compound will be
a difference of zero will be purely covalent

Question 14

covalent bonds and polarity

Answer 14

covalent bonds can become polar if the atoms attached to it have a difference in electronegativity
the bigger difference in electronegativity, the more polar a bond will be

Question 15

what causes polarity?

Answer 15

uneven distribution of charge
e.g. H2O
(symmetrical arrangements creates no overall polarity e.g. CO2)

Question 16

intermolecular forces - in order of strength

Answer 16

hydrogen bonding
permanent dipole-dipole
induced dipole-dipole

Question 17

basics of induced dipole-dipole fores
iodine example

Answer 17

can hold some molecules in crystal structures
e.g. iodine (I2)
strong covalent bonds hold the two iodine atoms together and WEAK induced dipole-dipole forces hold the I2 molecules together
*the bigger the molecule or atom, the more induced dipole-dipole forces as you have larger electron clouds

Question 18

long/straight hydrocarbons and induced dipole-dipole forces

Answer 18

longer (more electrons = more induced dipole-dipoles)), straight chain hydrocarbons have more induced dipole-dipole forces and so more energy is needed to overcome these forces, boiling point increases

Question 19

branched hydrocarbons and induced dipole-dipole forces

Answer 19

more branches means the molecules cant pack closely together (less points of surface area contact) which weakens the induced dipole-dipole forces between the chains and lowers their boiling point

Question 20

permanent dipole-dipole forces

Answer 20

weak electrostatic forces
exist between molecules with polarity
involves molecules with a permanent dipole
stronger

Question 21

permanent and induced relationship

Answer 21

molecules that have permanent dipole-dipole interactions also have induced dipole-dipole interactions too

Question 22

how can polar molecules be tested

Answer 22

place charged rod near a steady stream of polar liquid
liquid should bend towards rod as molecules align to face the oppositely charged rod

Question 23

hydrogen bonding

Answer 23

the strongest intermolecular force that occurs when you have very electronegative elements:
N, O, F
hydrogen bonds shown by dotted lines between lone pairs and hydrogen

Question 24

ice

Answer 24

• regular structure held by hydrogen bonding
• water molecules further apart in ice than when in liquid form
• ice is less dense than water

Question 25

boiling points in hydrogen halides

Answer 25

1. HF has a higher boiling point than HCl as it has hydrogen bonding
   more energy is needed to overcome electrostatic forces
2. slight increase in boiling point from HCl to HI due to increased mass of molecule, hence bigger electron cloud and more induced dipole-dipole forces

Question 26

simple molecular/simple covalent summary

Answer 26

e.g. I2. NH3, H2O
RTP: usually liquid or gas (except iodine)
conduct electricity: not as solid or liquid
solubility: depends on polarity of molecule
melting and boiling point: low due to weak forces breaking NOT bonds

Question 27

giant ionic summary

Answer 27

e.g. NaCl, CaO, MgBr2
RTP: solid
conduct electricity: not as solid, but does as a liquid due to free ions
solubility: yes
melting and boiling point: high due to strong electrostatic forces