3.6- THE SHAPE OF MOLECULES AND IONS

Question 1

What kind of dimension do molecules have?

Answer 1

three-dimensional

Question 2

What sorts of shapes do molecules come in?

Answer 2

may different shapes

Question 3

How do electrons in molecules exist?

Answer 3

in pairs in volumes of space called orbitals

Question 4

What ideas can you use to predict the shape of a simple covalent molecule e.g. one consisting of a central atom surrounded by a number of other atoms? (2)

Answer 4

each pair of electrons around an atom will repel all other electron pairs

pairs of electrons will take up positions as far apart as possible to minimise repulsion

Question 5

What is being able to predict the shape of a simple covalent molecule using ideas: each pair of electrons around an atom will repel all other electron pairs + pairs of electrons will take up positions as far apart as possible to minimise repulsion called?

Answer 5

electron pair repulsion theory

Question 6

How can electron pairs be?

Answer 6

be a shared pair or a lone pair

Question 7

What does the shape of a simple molecule depend on?

Answer 7

number of pairs of electrons that surround central atom

Question 8

What do you need to do first when trying to work out the shape of any molecule?

Answer 8

draw a dot-and-cross diagram to find the number of pairs of electrons

Question 9

What will the shape of the molecule be if there are two pairs of electrons around the central atom?

Answer 9

molecule will be linear

Question 10

What is the furthest away from each other that the two pairs can get? (two pairs of electrons)

Answer 10

180°

Question 11

What is an example of molecule that has two pairs of electrons that surround the atom? (two pairs of electrons)

Answer 11

beryllium chloride

Question 12

When is beryllium chloride covalently bonded molecule? (two pairs of electrons)

Answer 12

in the gas phase despite being a metal- non-metal compound

Question 13

How far apart would they be if there are three pairs of electrons around the central atom?

Answer 13

120° apart

Question 14

What would the shape of the molecule be like if there are three pairs of electrons around the central atom?

Answer 14

molecule is planar and is called trigonal planar

Question 15

What is an example of a molecule that has three pairs of electrons around the central atom?

Answer 15

boron trifluoride

Question 16

If there are four pairs of electrons, when are they the furthest apart?

Answer 16

when they are arranged so that they point to the four corners of a tetrahedron

Question 17

Where is one of the atoms positioned? (four pairs of electrons)

Answer 17

at the centre

Question 18

what is the shape of the molecule with four pairs of electrons?

Answer 18

tetrahedral

Question 19

What is an example of a molecule with four pairs of electrons?

Answer 19

methane

Question 20

Where is the carbon atom of methane situated? (four pairs of electrons)

Answer 20

at the centre of the tetrahedron

Question 21

Where are the hydrogen atoms of methane situated? (four pairs of electrons)

Answer 21

at the vertices

Question 22

What are the angles like? (four pairs of electrons)

Answer 22

109.5°

Question 23

What is the shape of the molecule like with four pairs of electrons?

Answer 23

three-dimensional, not planar

Question 24

As the molecule is not three-dimensional, how does this affect the sum of the angles? (four pairs of electrons)

Answer 24

sum of angles can be more than 360°

Question 25

What ion is tetrahedral?

Answer 25

ammonium ion

Question 26

Why is the ammonium ion tetrahedral?

Answer 26

four groups of electrons surrounding the nitrogen atom

Question 27

Does the ammonium ion having an overall charge affect the shape?

Answer 27

no

Question 28

What is the shape usually when there are five pairs of electrons?

Answer 28

trigonal bipyramid

Question 29

What is an example of a molecule with five pairs of electrons?

Answer 29

phosphorus pentachloride, PCl5

Question 30

What shape is adopted if there are six pairs of electrons?

Answer 30

octahedral

Question 31

What are the bond angles when there are six pairs of electrons?

Answer 31

90°

Question 32

What is an example of a molecule with six pairs of electrons?

Answer 32

sulphur hexafluoride, SF6

Question 33

what do some molecules have? (hint- electrons)

Answer 33

unshared (lone) pairs of electrons

Question 34

What are unshared (lone) pairs of electrons?

Answer 34

electrons that are not part of a covalent bond

Question 35

Examples of molecules where lone pairs affect shape? (2)

Answer 35

ammonia | water

Question 36

How many pairs of shared and unshared electrons does ammonia have?

Answer 36

four pairs of electrons and one of the groups is a lone pair

Question 37

With its four pairs of electrons around the nitrogen atom, what is the shape of the ammonia molecule based on?

Answer 37

a tetrahedron

Question 38

As there are only three 'arms' in ammonia what is the actual shape of ammonia?

Answer 38

triangular pyramid

Question 39

Where are the bonding pairs of electrons attracted towards in ammonia?

Answer 39

towards nitrogen nucleus + hydrogen nucleus

Question 40

What is the lone pair of electrons in ammonia attracted by and what happens?

Answer 40

attracted only by nitrogen nucleus and so is pulled closer to it than shared pairs

Question 41

As the lone pair on ammonia is pulled closer to the nitrogen than the shared pairs, what is the repulsion like?

Answer 41

repulsion between lone pair of electrons + bonding pair of electrons greater than that between two bonding pairs

Question 42

What does the effect of increased repulsion between the lone pair and bonding pairs of electrons in ammonia have?

Answer 42

squeezes hydrogen atoms together, reducing all H-N-H angles

Question 43

What is the approximate rule of thumb per lone pair? (hint-an angle)

Answer 43

2°

Question 44

What is the bond angles in ammonia approximately?

Answer 44

107°

Question 45

How many pairs of electrons are there around the oxygen atom in water?

Answer 45

four pairs of electrons

Question 46

What is the shape of water based on?

Answer 46

tetrahedron

Question 47

How many 'arms' of the tetrahedron are lone pairs that are not part of a bond? (water)

Answer 47

two

Question 48

As two of the 'arms' of the tetrahedron are lone pairs that are not part of a bond what shape does this result in? (water)

Answer 48

V-shaped or angular molecule

Question 49

What shape do the electrons form in water? (same as ammonia)

Answer 49

tetrahedron

Question 50

What shape do the bonds form in water?

Answer 50

V-shaped

Question 51

What is the H-O-H angle? (water)

Answer 51

104.5°

Question 52

What shape does the electrons form in ammonia?

Answer 52

tetrahedron

Question 53

What shape do the bonds form in ammonia?

Answer 53

triangular pyramid

Question 54

How many bonding pairs and lone pairs are there in chlorine tetrafluoride ion, CLF4-?

Answer 54

four bonding pairs | two lone pairs

Question 55

What do one of the lone pairs in chlorine tetrafluoride ion, CLF4- contain?

Answer 55

electron that has been donated to it, so charge on ion is negative (-1)

Question 56

How many pairs of electrons are there around the chlorine atom in chlorine tetrafluoride ion, CLF4-?

Answer 56

six pairs of electrons

Question 57

What is the shape of chlorine tetrafluoride ion, CLF4- based on?

Answer 57

octahedron in which two arms are not part of a bond

Question 58

As lone pairs repel what position do they adopt? (chlorine tetrafluoride ion, CLF4-)

Answer 58

adopt a position furthest apart

Question 59

What shape does it leave as the lone pairs adopt a position furthest apart? (chlorine tetrafluoride ion, CLF4-)

Answer 59

flat square-shaped ion described as square planar

Question 60

Bonding repulsion from lowest to highest:

Answer 60

bonding pair- bonding pair lone pair- bonding pair lone pair- lone pair