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### Homework Question 4.1 (6th edition)

Posted: Wed Nov 14, 2018 2:39 pm
Q: Below are ball-and-stick models of two molecules. In each case, indicate whether there must be, may be, or cannot be one or more lone pairs of electrons on the central atom.
120° (a)
180° (b)

How do you know if there can be lone pairs of electrons on the central atom? Wouldn't it be another shape (not linear) if there was lone pairs on the central atom?

### Re: Homework Question 4.1 (6th edition)

Posted: Wed Nov 14, 2018 3:14 pm
You're right! The shape wouldn't be linear if there was a/were lone pair(s) on the central atom. That's why for a) where the bond angle is 120 degrees, it is possible for there to be lone pairs on the central atom since the shape of the molecule is bent (repulsion from the lone pair(s) push the two bonded atoms down). For b) however, since it is 180 degrees and linear, then it cannot have any lone pairs on the central atom.

### Re: Homework Question 4.1 (6th edition)

Posted: Wed Nov 14, 2018 3:26 pm
Actually, in discussion today, we learned about T shape molecules. IT is possible to have 2 or 3 or even 4 lone pairs around the central atom and then 2 other atoms connected to it to form a linear shape. So part b) would be that it MAY be possible to have lone pairs.

### Re: Homework Question 4.1 (6th edition)

Posted: Thu Nov 15, 2018 12:11 am
In response to the above post, I don't believe it is possible for the model in part (b) to have two lone pairs. If you think about it, if there were two lone pairs, the electron geometry would be a tetrahedral. Therefore, the two bonds would have a bond angle of slightly less than 109.5 degrees. The VSEPR formula for a molecule with two bonds and two lone pairs would be AX2E2 which means the molecular geometry would be bent. Therefore, it's impossible for the model in part (b) to have two lone pairs. However, I do agree that it may have 3 or 4 lone pairs. If the VSEPR formula for the molecule looked like AX2E3, the molecular geometry would be liner and the electron geometry would be trigonal bipyramidal. If the VSEPR formula for the molecule looked like AX2E4, the molecular geometry would be linear and the electron geometry would be octahedral.

### Re: Homework Question 4.1 (6th edition)

Posted: Thu Nov 15, 2018 2:13 am
But for b), couldn't there be lone pairs that repel each other. For example, one lone pair at the top of the central atom and one at the bottom. However, they would not have an effect on the bond angle since the repulsion forces from the lone pairs cancel each other out.