[sveioen]

Hi all,

I'm trying to figure out the geometry of N(CH₃)₃.First of all, when I have been asked to describe the geometry of N(CH₃)₃, do I need to analyse each "central atom"? In this case N and the 3 C-atoms, or just the N-atom?.

I have drawn Lewis-structure and of course the Nitrogen atom has one lone pair and 3 bonding pairs of electrons. This would give a tetrahedral, but since the lone pair is "invisible", this is really a trigonal pyramid. Is this correct? Also, the lone pair causes the angle between the C-bondings to be slightly less than 109.5 degrees, right?

[Dan]

I'm trying to figure out the geometry of N(CH₃)₃.First of all, when I have been asked to describe the geometry of N(CH₃)₃, do I need to analyse each "central atom"? In this case N and the 3 C-atoms, or just the N-atom?

Probably just the N

I have drawn Lewis-structure and of course the Nitrogen atom has one lone pair and 3 bonding pairs of electrons. This would give a tetrahedral,

Correct

but since the lone pair is "invisible", this is really a trigonal pyramid. Is this correct? Also, the lone pair causes the angle between the C-bondings to be slightly less than 109.5 degrees, right?

Agreed

[sveioen]

Thanks for replying Dan! I guess I got it more or less correct.

However, when you're asked to tell about the geometry of a molecule, do you count the lone pair? Is this a tetrahedral or trigonal pyramid? It appears to me that you first count all electrons around the central atom which gives you the general geometry, and then correct for the lone pairs. So instead of AX₄E₀, this is a AX₃E₁. So the correct answer to this question would be that it is a trigonal pyramid. Is this a correct conclusion? Or could you call this a tetrahedral as well?

[Dan]

So the correct answer to this question would be that it is a trigonal pyramid. Is this a correct conclusion? Or could you call this a tetrahedral as well?

I would say the geometry is trigonal pyramidal - if you're going count the lone pair and call it tetrahedral then you would also call water tetrahedral, and even HCl tetrahedral (3 lone pairs and one bonding pair on the Cl) which generally isn't done.

The symmetry of the orbitals is tetrahedral (well, the atomic orbitals on the central atom) but as long as the question is about the geometry of the molecule then usually only the atoms (or substituents - like CH₃) are considered when naming the shape. To look at it another way, the lone pairs affect the shape but are not part of it for the purposes of this kind of description.

[sveioen]

Thanks a lot Dan, I'm starting to get this.