ikillmcatz MCAT Questions

[ikillmcatz]

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[circulus vitios]

All my questions will be listed in this thread as I embark upon my journey to acing the MCAT. I truly appreciate you guys for answering my questions/helping me work through this material.

First: How do you guys figure out what the electron configurations are for ions? I'm thoroughly confused by it now, even though when I took the class I understood it pretty well....
For ex: if the electron config of something is 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5, how do you know which ion this is?????

 

[darkjedi]

This might help you too, in order to determine in what order each shell is filled.

If you count up the total number of electrons that should give you an idea of what element it is, you'd need to know the charge as well to account for missing/extra electrons.

 

[ikillmcatz]

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[ikillmcatz]

Huh this is interesting...makes sense...is this just something you would have to intuitively know?

EDIT: But see how there are two different ones for 3d5? How do know which one of those is the right answer? Because I have both of those elements/ions in my answer choices but can only pick one...

 

[circulus vitios]

Huh this is interesting...makes sense...is this just something you would have to intuitively know?

EDIT: But see how there are two different ones for 3d5? How do know which one of those is the right answer? Because I have both of those elements/ions in my answer choices but can only pick one...

There are two different ones for 3d5 because that's the most stable configuration. Chromium would much rather have a 4s1 3d5 configuration (both orbitals being half full) than 4s2 3d4 (the 4s orbital being full and the 3d orbital having two unpaired spins.) The same applies to molybdenum: 5s1 4d5 is more stable than 5s2 4d4. I'm not sure why tungsten and seaborgium are an exception to this rule.

Back to your original question: it's been forever since I've looked at general chemistry but I think they would have to give you some additional information. If they told you the element or ion was neutrally charged, then 1s2 2s2 2p6 3s2 3p6 4s1 3d5 would be chromium.

 

[ikillmcatz]

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[Lunasly]

Quick question for you guys. Some sources claim that Tungsten is not one of the exceptions like Cr and Mo. However, BR says it is. Can anyone confirm which it is?

 

[Lunasly]

Anyone?

 

[DrknoSDN]

Yes, multiple sources confirm that Tungsten obeys the aufbau principle.

 

[BerkReviewTeach]

And multiple sources confirm that Tungsten does not obey the aufbau principle, such as Levine as well as Chang. When there is ambiguity between different sources, and this happens with many things in science, I assume the MCAT test writers will not ask a question that requires one ideology to be chosen over another.

Given that electronic configurations are adventures in book-keeping, know your basics, a few exceptions like Cr and Cu, and know how to work with ions and you'll be fine.

 

[Lunasly]

Thanks for the clarification!