Ch 3: Shapes and Polarity of Chemical Species Flashcards
Why is the shape of chemical species important?
it accounts for many of the macroscopic properties of real-world objects
How can the 3D shapes of molecules and molecular ions of the p-block elements (ie. O, C, N,B, P, etc.) be predicted?
using a simple theory built upon electron-electron repulsion
What is the VSEPR theory?
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
How do you represent a bond that is oriented out of the page?
solid wedge
How do you represent a bond that is oriented behind the plane of the page?
dashed wedge
What is a perspective diagram?
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
What is the simplest species for which the shape can be predicted?
those in which one central atom is surrounded by lone pairs and at least two bonds to terminal atoms
How do you determine molecular shape using the VSEPR model?
- draw the best Lewis structure
- 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 - molecular shape is determined by considering the relative number of lone pairs (E) and atoms (X) bonded to the central atom (A)
What is the parent shape?
lowest energy arrangement of all lone and bonding electron pairs, which minimizes electrostatic repulsion in 3D
What is the molecular shape of all diatomic molecules?
linear
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)
linear
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)
trigonal planar
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)
tetrahedral
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)
trigonal bipyramidal
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)
octahedral
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)
pentagonal bipyramidal
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)
E: 0
X: 2
molecular shape: linear
angle: 180
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)
E: 0
X: 3
molecular shape: trigonal planar
angle: 120
AND
E: 1
X: 2
molecular shape: bent
angle: 120
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)
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
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)
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)
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)
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
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)
E: 0
X: 7
molecular shape: pentagonal bipyramidal
angle: 72 (eq-eq), 90 (ax-eq), 180 (ax-ax)
How do you determine polarity?
- determine the molecular shape and draw a perspective diagram
- on the perspective diagram, draw bond dipoles for all of the bonds, indicating both direction and relative magnitude of each bond dipole
- 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
What needs to be considered to determine if a molecule will have an overall dipole moment?
in molecules with more than one atoms, both the polarity of the bonds and the shape of the molecule must be considered
