Shapes of molecules

Question 1

How to work out shapes of molecules with single bonds only

Answer 1

1. Find central atom and work out group number
2. Add no. atoms to group number
3. If ion is positively charged subtract that charge, if negatively charged then add the charge
4. Add up all electrons and divide by 2 to get number of electron pairs
5. Lone pairs = electron pairs - number of bonded groups

Question 2

What does covalent bonds are directional in space mean?

Answer 2

Molecules have a set shape depending on the number of electron pairs around the central atom. The outer electrons of an atom in a molecule are arranged in pairs.

Question 3

What can electron pairs be thought as?

Answer 3

Clouds of electron density which repel each other so that they are as far apart as possible. An electron pair may be a shared pair of electrons or a lone pair of electrons

Question 4

Electron pair repulsion theory

Answer 4

1) Electron pairs repel as far away as possible
2) Lone pair / lone pair repulsion is greater than any lone pair / bonding pair repulsion which is greater than bonding pair / bonding pair repulsion

Where a molecule has no lone pair of electrons, number of bonding pairs repel equally

Question 5

Linear e.g. BeCl2

Answer 5

Bond angle = 180 degrees
Electron pairs = 2
Bonding pairs = 2
Lone pairs = 0 
Symmetric

Question 6

Trigonal planar e.g. BeCl3

Answer 6

Bond angle = 120 degrees 
Ep = 3
Bp = 3
Lp = 0 
Symmetric

Question 7

Tetrahedral e.g. NH4+ or CH4

Answer 7

Bond angle = 109.5 degrees
Ep = 4 
Bp = 4
Lp = 0
Symmetric

Question 8

Pyramidal e.g. NH3 or PH3

Answer 8

Bond angle = 107 degrees
Ep = 4 
Bp = 3
Lp = 1
Asymmetric

Question 9

V or bend shape e.g. H2O

Answer 9

Bond angle = 104.5 degrees 
Ep = 4
Bp = 2
Lp = 2
Asymmetric

Question 10

Trigonal bypyramidal e.g. PF5

Answer 10

Bond angle = 120 and 90 degrees
Ep = 5
Bp = 5
Lp = 0

Question 11

Modified trigonal bypyramidal e.g. SF4

Answer 11

Bond angle = 188 and 89 degrees
Ep = 5
Bp = 4
Lp = 1

Question 12

T shaped e.g. ClF3

Answer 12

Bond angle = 86 degrees
Ep = 5
Bp = 3
Lp = 2

Question 13

Linear e.g. I3-

Answer 13

Bond angle = 180 degrees
Ep = 5
Bp = 2
Lp = 3

Question 14

Octahedral e.g. SF6

Answer 14

Bond angle = 90 degrees
Ep = 6
Bp = 6
Lp = 0 
Symmetric

Question 15

Square planar e.g. ICl4

Answer 15

Bond angle = 90 degrees
Ep = 6
Bp = 4
Lp = 2

Question 16

How would you know that a molecule has a linear shape? Give the bond angle.

Answer 16

Molecules with TWO ELECTRON PAIRS
Bond angle: 180 degrees
The pairs of bonding electrons want to be far away from each other as possible.
e.g. BeCl2

Question 17

How would you know that a molecule has a trigonal planar shape? Give the bond angle.

Answer 17

Molecules with THREE ELECTRON PAIRS - and there are THREE BONDING PAIRS - repulsion of charge clouds is the same between each pair.
Bond angle: 120 degrees
e.g. BF3

Question 18

How would know that a molecule has a tetrahedral shape? Give the bond angle.

Answer 18

FOUR ELECTRON PAIRS and FOUR BONDING PAIRS. no lone pairs, so that the charge clouds all repel each other equally.
Bond angle: 109.5 degrees
e.g. NH4+

Question 19

How would you know that a molecule has a trigonal pyramidal shape? Give the bond angle.

Answer 19

FOUR electron pairs and 3 BONDING PAIRS and 1 LONE PAIRS
the lone pair/bonding-pair repulsion will be greater than the bonding-pair/bonding-pair repulsion so angles of the atoms changes.
Bond angle is 107 degrees.
larger bond angle between lone and bonding pairs and smaller bond angle between bonding pairs.

Question 20

How would you know that a molecule has a bent / v-shaped / non-linear shape? Give the bond angle.

Answer 20

FOUR electron pairs and 2 BONDING PAIRS and 2 LONE PAIRS.
The lone-pair/lone-pair repulsion will squish the bond angle even further.
Bond angle: 104.5 degrees
e.g. H2O

Question 21

How would you know that a molecule has a trigonal bipyramidal shape? Give the bond angle.

Answer 21

FIVE electron pairs and 5 BONDING PAIRS, no lone pairs.
Repulsion between bonding pairs means that three of the atoms will form a trigonal planar shape with bond angles of 120 degrees and the other two will be at 90 degrees.
Bond angles: 120 and 90 degrees
e.g. PCl5

Question 22

How would you know that a molecule has a seesaw shape? Give the bond angle.

Answer 22

FIVE electron pairs: FOUR BONDING PAIRS and 1 LONE PAIR.
The lone pair is always positioned where one of the trigonal planar atoms would be in a trigonal bipyramidal molecule.
e.g. SF4 - bond angle: 102 and 86.5 degrees

Question 23

How would you know that a molecule has a T-shape? Give the bond angle.

Answer 23

FIVE electron pairs: 3 BONDING PAIRS and 2 LONE PAIRS.
e.g. ClF3 - central Cl has 3 bonding and 2 lone pairs.
Bond angle: 87.5 degrees

Question 24

How would you know that a molecule has an Octahedral shape? Give the bond angle.

Answer 24

SIX electron pairs: 6 BONDING PAIRS and no lone pairs.
e.g. SF6 - central atom has six bonding pairs and no lone pairs.
Bond angle: All bonds 90 degrees

Question 25

How would you know that a molecule has a square pyramidal shape? Give the bond angle.

Answer 25

SIX electron pairs: 5 BONDING PAIRS and 1 LONE PAIR
e.g. ClF5
Bond angle: 90 degrees

Question 26

How would you know that a molecule has a square planar shape? Give the bond angle.

Answer 26

SIX electron pairs: 4 BONDING PAIRS and 2 LONE PAIRS
e.g. XeF4
Bond angle: 90 degrees

Question 27

What shape is carbon dioxide? (CO2)

Answer 27

Linear
It has four bonding pairs of electrons (found in two sulfur-oxygen double bonds) and no lone pairs.
Double bonds can be treated as one bond - two bonds no lone pairs.
Bond angle: 180 degrees

Question 28

What shape is sulfur dioxide? (SO2)

Answer 28

Bent / v-shaped / non-linear
It has four bonding pairs of electrons and one lone pair It has two double bonds that include the four bonding pairs.
Bond angle: 120 degrees