Ch 3: Shapes and Polarity of Chemical Species

Question 1

Why is the shape of chemical species important?

Answer 1

it accounts for many of the macroscopic properties of real-world objects

Question 2

How can the 3D shapes of molecules and molecular ions of the p-block elements (ie. O, C, N,B, P, etc.) be predicted?

Answer 2

using a simple theory built upon electron-electron repulsion

Question 3

What is the VSEPR theory?

Answer 3

valence-shell electron-pair repulsion theory

non-bonding and bonding electron pairs in the valence shell of the central atom distribute as dar apart as possible in 3D space due to electrostatic repulsion
- this orientation minimizes the net electrostatic repulsion, which means the overall energy of the molecule is minimized

Question 4

How do you represent a bond that is oriented out of the page?

Answer 4

solid wedge

Question 5

How do you represent a bond that is oriented behind the plane of the page?

Answer 5

dashed wedge

Question 6

What is a perspective diagram?

Answer 6

an arrangement that attempts to represent the 3D arrangement of atoms and bonds in two dimensions by showing bonds oriented out of the page (solid wedge) or behind the page (dashed wedge)

• bonds drawn as straight lines indicate that they all in the plane of the paper

Question 7

What is the simplest species for which the shape can be predicted?

Answer 7

those in which one central atom is surrounded by lone pairs and at least two bonds to terminal atoms

Question 8

How do you determine molecular shape using the VSEPR model?

Answer 8

1. draw the best Lewis structure
2. sum the number of lone pairs of electrons (E) and the number of attached atoms (X) to determine the parent shape
   - X is not necessarily the same as the total number of bonds (ie. we do not distinguish single vs. double or triple bonds
3. molecular shape is determined by considering the relative number of lone pairs (E) and atoms (X) bonded to the central atom (A)

Question 9

What is the parent shape?

Answer 9

lowest energy arrangement of all lone and bonding electron pairs, which minimizes electrostatic repulsion in 3D

Question 10

What is the molecular shape of all diatomic molecules?

Answer 10

linear

Question 11

sum of lone pairs and attached atoms (E+X): 2

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 11

linear

Question 12

sum of lone pairs and attached atoms (E+X): 3

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 12

trigonal planar

Question 13

sum of lone pairs and attached atoms (E+X): 4

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 13

tetrahedral

Question 14

sum of lone pairs and attached atoms (E+X): 5

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 14

trigonal bipyramidal

Question 15

sum of lone pairs and attached atoms (E+X): 6

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 15

octahedral

Question 16

sum of lone pairs and attached atoms (E+X): 7

parent shape?
look at 3D shape and model in table 3.1 (pg. 3.4)

Answer 16

pentagonal bipyramidal

Question 17

PARENT SHAPE: linear

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 17

E: 0
X: 2
molecular shape: linear
angle: 180

Question 18

PARENT SHAPE: trigonal planar

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 18

E: 0
X: 3
molecular shape: trigonal planar
angle: 120

AND

E: 1
X: 2
molecular shape: bent
angle: 120

Question 19

PARENT SHAPE: tetrahedral

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 19

E: 0
X: 4
molecular shape: tetrahedral
angle: 109.5

AND

E: 1
X: 3
molecular shape: trigonal pyramidal
angle: 109.5

AND

E: 2
X: 2
molecular shape: bent
angle: 109.5

Question 20

PARENT SHAPE: trigonal bipyramidal

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 20

E: 0
X: 5
molecular shape: trigonal bypyramidal
angle: 90 (ax-eq), 120 (eq-eq), 180 (ax-ax)

AND

E: 1
X: 4
molecular shape: seesaw
angle: 90 (ax-eq), 120 (eq-eq), 180 (ax-ax)

AND

E: 2
X: 3
molecular shape: T-shaped
angle: 90 (ax-eq), 180 (ax-ax)

AND

E: 3
X: 2
molecular shape: linear
angle: 180 (ax-ax)

Question 21

PARENT SHAPE: octahedral

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 21

E: 0
X: 6
molecular shape: octahedral
angle: 90 and 180

AND

E: 1
X: 5
molecular shape: square-based pyramidal
angle: 90 and 180

AND

E: 2
X: 4
molecular shape: square planar
angle: 90 and 180

Question 22

PARENT SHAPE: pentagonal bipyramidal

E (lone pairs)?
X (attached atoms)?
molecular shape?
predicted approximate X-A-X bond angle(s)?

look at perspective diagram and 3D model in table 3.2 (pg. 3.5)

Answer 22

E: 0
X: 7
molecular shape: pentagonal bipyramidal
angle: 72 (eq-eq), 90 (ax-eq), 180 (ax-ax)

Question 23

How do you determine polarity?

Answer 23

1. determine the molecular shape and draw a perspective diagram
2. on the perspective diagram, draw bond dipoles for all of the bonds, indicating both direction and relative magnitude of each bond dipole
3. sum the dipole vectors:
   - if the sum is 0: non-polar
   - if the sum is non-zero: polar and the molecule has a net dipole moment which can be represented by a vector arrow

Question 24

What needs to be considered to determine if a molecule will have an overall dipole moment?

Answer 24

in molecules with more than one atoms, both the polarity of the bonds and the shape of the molecule must be considered