Shapes of Molecules and Ions

Question 1

according to electron pair repulsion theory, what is the shape of a molecule or ion caused by

Answer 1

• the repulsion between the pairs of electrons
• (both bond pairs and non-bonded pairs)
• that surround the central atom

Question 2

how do electrons arrange themselves in a molecule or ion

Answer 2

• they arrange themselves around the central atom

- so that the repulsion between them is a minimum

Question 3

how does the strength of the repulsion between the electrons differ according to whether it is an interaction between a lone pair or bonded pair

Answer 3

• lone pair to lone pair repulsion is the strongest
• followed by lone pair to bond pair repulsion
• then bond pair to bond pair repulsion is the weakest

Question 4

what does VSEPR stand for

Answer 4

valence shell electron pair repulsion (theory)

Question 5

what are the three rules usually used for

Answer 5

• rule 1 and 2 are used to obtain the basic shape of a molecule or ion
• rule 3 is used to estimate the values for the bond angles

Question 6

what is the first thing you need to work out if you want to determine the shape of a molecule

Answer 6

the number of lone pairs and bonded pairs of electrons around the central atom

Question 7

how would this be done easily

Answer 7

by drawing a dot and cross diagram

Question 8

if a molecule had multiple bonds between two atoms, like a double bond, how should you treat that multiple bond

Answer 8

like it is only one pair of electrons

Question 9

using that knowledge, how many pairs of electrons are around the C in O=C=O

Answer 9

2

Question 10

if a double bond is treated like an electron pair, what is the shape of a CO2 molecule and why

Answer 10

• its a linear molecule

- because carbon doesnt have any lone pairs of electrons due to the double bonds

Question 11

what is the shape, number of bond pairs and lone pairs for BeCl2

Answer 11

• linear
• 2 bond pairs
• 0 lone pairs

Question 12

what is the shape, number of bond pairs and lone pairs for BCl3

Answer 12

• trigonal planar
• 3 bond pairs
• 0 lone pairs

Question 13

what is the shape, number of bond pairs and lone pairs for CH4

Answer 13

• tetrahedral
• 4 bond pairs
• 0 lone pairs

Question 14

what is the shape, number of bond pairs and lone pairs for PCl5

Answer 14

• trigonal bipyramidal
• 5 bond pairs
• 0 lone pairs

Question 15

what is the shape, number of bond pairs and lone pairs for NH3

Answer 15

• trigonal pyramidal
• 3 bond pairs
• 1 lone pair

Question 16

what is the shape, number of bond pairs and lone pairs for H2O

Answer 16

• v-shaped
• 2 bond pairs
• 2 lone pairs

Question 17

what is the shape of sulfur trioxide and why

Answer 17

• sulfur has the oxidation number of +6
• an oxygen atom has the oxidation number -2
• meaning that for a neutral molecule its formula would be SO3
• this also means that there are double bonds between the sulfur and each oxygen
• however, double bond can be treated as one bonded pair of electrons
• and as there are no lone pairs of electrons, its shape would be trigonal planar (like BCl3)

Question 18

what is the bond angle for a linear molecule

Answer 18

180

Question 19

what is the bond angle for a trigonal planar molecule

Answer 19

120

Question 20

what is the bond angle for a tetrahedral molecule

Answer 20

109.5

Question 21

using the bond angle for tetrahedral molecules as the base angle, what does a lone pair of electrons in a molecule do to the bond angles

Answer 21

it reduces them by 2.5 degrees

Question 22

what is the bond angle for the trigonal pyramidal molecules like NH3 and why

Answer 22

• 107
• N has 5 electrons in its outer shell whereas H has 1
• this means that N would have 2 electrons left after bonding, aka a lone pair
• they reduce the base angle of 109.5 by 2.5 making it 107

Question 23

what is the bond angle for v-shaped molecules like H2O and why

Answer 23

• 104.5
• oxygen has 6 electrons in its outer shell and H has 1
• after bonding O would have 4 electrons aka 2 lone pairs
• joint together they reduce the angle by 5 from 109.5
• making it 104.5

Question 24

what are the bond angles for trigonal bipyramidal molecules and why

Answer 24

• 90 and 120
• they would have 3 atoms bonded to the centre atom in one plane (horizontal)
• meaning they would be dispersed evenly at 360 / 3 = 120 each
• then the last two are bonded at the top and bottom on the same plane (vertical)
• making the angle between one of the ones in the vertical and the horizontal 90

Question 25

what are the bond angles for hexagonal molecules and why

Answer 25

• 180 and 90
• 4 atoms are bonded to the centre atom in the same horizontal plane
• so the angle between all of them would be 90
• then there are two atoms on the top and bottom of the centre atom in the vertical plane
• the angle between them is 180 (dont ask me why it doesnt apply to the previous one)