Guess the central atom...

[SaintJude]

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Say you were given the formula POCl3. Don't go googling!!

How do you (quickly) know whether Oxygen or Phosphorus is the central atom ?

 

[pfaction]

It's definitely not Cl3. TPR says to choose the least electronegative atom as the center. I don't know how reliable that is.

 

[chiddler]

Say you were given the formula POCl3. Don't go googling!!

How do you (quickly) know whether Oxygen or Phosphorus is the central atom ?

Think about how many bonds each can have. O can bind two things. Cl one. P on the other hand....

 

[MedPR]

Say you were given the formula POCl3. Don't go googling!!

How do you (quickly) know whether Oxygen or Phosphorus is the central atom ?

Phosphorous because it is in the same column as Nitrogen and Nitrogen can form 4 bonds.

 

[SaintJude]

Yeah, Kaplan says the same thing : choose the least electronegative atom. What? But Kaplan reserves most of its knowledge on the online materials as you go through explanations.

How was I supposed to know that?

Edit: And: "Phosphorus violates the octet rule" All of these random rules.. In fact, this is how it looks like...That would have taken me forever to figure out. So is this trigonal pyramidal?

 

[pfaction]

But remember, it has an extended valence shell. It's in the 3 so it can have a lot more than 8 electrons.

 

[MedPR]

Yeah, Kaplan says the same thing : choose the least electronegative atom. What? But Kaplan reserves most of its knowledge on the online materials as you go through explanations.

How was I supposed to know that?

Edit: And: "Phosphorus violates the octet rule" All of these random rules.. In fact, this is how it looks like...That would have taken me forever to figure out. So is this trigonal pyramidal?

Just remember that oxygen, hydrogen, and halides are the last place you are going to find exceptions.

 

[pfaction]

No, it's still tetrahedral. Double bonds are ONE electron group. So POCl3 has 4 groups -> tetrahedral geometry and shape.
Trig pyramid is 3 e groups + 1 free. That'd be like NH3.

 

[MedPR]

No, it's still tetrahedral. Double bonds are ONE electron group. So POCl3 has 4 groups -> tetrahedral geometry and shape.
Trig pyramid is 3 e groups + 1 free. That'd be like NH3.

NH3 has tetrahedral electron geometry, trigonal pyramidal molecular geometry.

In electron geometry you count lone pairs as an electron domain. In molecular geometry you only count molecules as electron domains.

For example, H2O is both tetrahedral and bent.

POCl3 is still tetrahedral either way though. I don't know if trigonal pyramidal exists in electron geometry.

 

[danderson]

Phosphorous because it is in the same column as Nitrogen and Nitrogen can form 4 bonds.

I thought Nitrogen normally forms 3 bonds?

 

[MedPR]

I thought Nitrogen normally forms 3 bonds?

Ammonium.

 

[danderson]

Ammonium.

I was referring to how nitrogen follows the octet rule and forms a cation when it has four bonds, versus phosphorus which behaves differently with it's valency because it's in a different period. Although, I guess none of that really matters for your main point.

 

[SaintJude]

Thanks everyone. I think I'll just make sure to remember that the least electronegative atoms is usually the central atom.

 

[ipodtouch]

Just remember that oxygen, hydrogen, and halides are the last place you are going to find exceptions.

Shouldn't carbon also be on the top of this list?

 

[danderson]

Thanks everyone. I think I'll just make sure to remember that the least electronegative atoms is usually the central atom.

Remembering the acronym HONC helps.

H - 1 bond
O - 2 bonds
N - 3 bonds
C - 4 bonds