Unit 1-Shape And Polyatomic molecules

Question 1

Which theory predicts the shapes of molecules and polyatomic ions?

Answer 1

Valence shell electron pair repulsion

VSEPR

Question 2

How does Valence shell electron pair repulsion (VSEPR) theory work?

Answer 2

Shapes can be predicted from the number of bonding electron pairs and the number of non-bonding electron pairs.

Question 3

How is the total of electron pairs calculated?

Answer 3

Electron pairs = (number of electrons on centre + number of bonded atoms)/2.

Add 1 for each negative charge

Remove 1 for each + charge

For NH₃

5 outer electrons on the central nitrogen atom (electron arrangement 2, 5) + 3 hydrogen atoms bonded.

5+3=(8)/2 = 4 (3 bonded and 1 lone pair)

Question 4

What arrangement of electron pairs would be found when the total number of electron pairs is 2, 3, 4, 5 and 6, respectively. i.e. what are the shapes that would be obtained?

Answer 4

linear, trigonal (trigonal planar), tetrahedral, trigonal bipyramidal and octahedral

Question 5

What arrangement of electron pairs would be found where we have:

i) 3 total electron pairs but 2 bonding pairs
ii) 4 total electron pairs but 3 bonding pairs
iii) 6 total electron pairs but 4 bonding pairs

Answer 5

i) Bent/angular
ii) Trigonal Pyramid
iii) Square planar

Question 6

Electron pairs will repel each other. They can be bonded or non-bonding (lone) pairs.

What is the order of repulsive effect?

Answer 6

non-bonded pair:non-bonded pair > non-bonded pair:bonded pair > bonded pair: bonded pair

Question 7

What effect do lone pairs have of the positions of bonded pairs?

Answer 7

They reduce the bond angle.

Question 8

What shape would Beryllium dichloride take?

Answer 8

Linear

Question 9

What shape would hydrogen oxide take?

Answer 9

Bent/angular/non-linear

Question 10

What shape would Boron trifluoride take?

Answer 10

Trigonal planar