Well, the ideal way to discover usability issues with with in-person user testing, so if you think you could arrange a session with, say, five or six participants, that would be best. You can get much, much richer data that way - some usability issues are very difficult to capture by Q&amp;A.

Still, questionnaires aren't useless - though they will be affected by response bias (as well as self-reporting issues, memory biases and 'response acquiescence', where participants will tend to agree with statement questions). What sorts of questions you ask will depend on the kind of application you're talking about, and issues you already expect to see, but some starting points are:


Did you always feel like you knew what to do and where to click?
Did you ever do something and got an unexpected result?
Did you feel you could trust the application and the organization behind it?
Did you find the application attractive?
Did this application act and feel like other, more familiar applications?
How quickly could you get what you wanted with this application?


As for designing the questionnaire, you might be interested in this article by Nokia on mobile questionnaires for smartphone apps.

What is best practice for UX surveys of a mobile application?
Shouldn't reaching an octet be any atom's "goal"?

However, I've recently learned about cases that are either expanding octets, or have lesser than "enough" electrons for an octet abiding. e.g.:

S in Sulfur hexafluoride (Expanding octet with a $\text{3d}$)



Source

B in Boron trifluoride (It's "hexet", instead of octet)



Source

However, both $\ce{SnCl2}$ and $\ce{SnCl4}$ are existent. The latter is explainable with octet, but not the former. Surprisingly, $\ce{SnCl2}$ is more stable! 


How?
Why is the phenomenon happening?




Dave pointed out that the octet nonabidingness is happening in the vapor phase. So I redirect the question to ask about the vapor phase; since that's what I'm looking for.
