2E. 1 Question

[Aashka Popat 1A]

The question is: 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.

In part a, the angular molecule has a 120 degree bond angle and the answer says it must have lone pairs, but I don't understand why it has to. In part b, the answer says that the linear molecule may have lone pairs, and I'm not sure why either. Can anyone explain these to me?

[jeffreygong1I]

If the first didn't have lone pairs then the shape wouldn't be bent-- it would be much more energy efficient to be linear. The second could have three lone pairs on it and have that shape, or it could have no lone pairs.

[mayarivers3I]

The first model must have lone pairs because electron repulsion is what's causing the shape to be bent. The second may have lone pairs because the shape of a molecule can be straight if there are a balanced amount of lone pairs.