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CHEM1 AS Help! Naming Structures, Drawing Etc

Okay in the Jan 2009 Q2)e It says Arsenic is in the same group as nitrogen. It forms the compound AsH3 Draw the shape of an AsH3 molecule, including any lone pairs of electrons. Name the shape made by its atoms.

I would DRAW AsH3 as a tetrahedral (because of the 3 bonds + lone pair) but i thought when naming you disregard the lone pair so it would be a trigonal planar.

Can someone please clear this up for me x
Reply 1
Avatar for chembob
chembob
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It's not planar.
Reply 2
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thecdon
17
Original post by Fabolous
Okay in the Jan 2009 Q2)e It says Arsenic is in the same group as nitrogen. It forms the compound AsH3 Draw the shape of an AsH3 molecule, including any lone pairs of electrons. Name the shape made by its atoms.

I would DRAW AsH3 as a tetrahedral (because of the 3 bonds + lone pair) but i thought when naming you disregard the lone pair so it would be a trigonal planar.

Can someone please clear this up for me x


It would be trigonal pyrimidal shape, with 4 bonding pairs or electrons, including one lone pair. This lone pair repels more than bonding pairs so therefore the bond angle would be 107 degrees.
The shape is not tetrahedral because that shape has bonding angles of 109.5 degrees and there are indeed 4 electron bonded pairs around the central atom but there are no lone pairs, so no repulsion. A tetrahedral molecule is methane or an ammonium ion.
Reply 3
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Fabolous
OP
Original post by thecdon
It would be trigonal pyrimidal shape, with 4 bonding pairs or electrons, including one lone pair. This lone pair repels more than bonding pairs so therefore the bond angle would be 107 degrees.
The shape is not tetrahedral because that shape has bonding angles of 109.5 degrees and there are indeed 4 electron bonded pairs around the central atom but there are no lone pairs, so no repulsion. A tetrahedral molecule is methane or an ammonium ion.


Hi, thanks for your help - so you would draw it as a trigonal pyrimidal shape but would you name it that?
The shape is tetrahedral, with the same bond angles as a tetrahedral but it would be called trigonal pyramidal.
For AS, this is sufficient.
Original post by WhatamIdoing
The shape is tetrahedral, with the same bond angles as a tetrahedral but it would be called trigonal pyramidal.
For AS, this is sufficient.


NO...

The SHAPE is trigonal pyramidal

The arrangement of electrons pairs/regions of electron density is tetrahedral.

You only refer to the shape of the molecule and the bond angles subtended between two atoms and the central atom. You do not refer to the arrangement of the electrons as the shape of the molecule.
Original post by charco
NO...

The SHAPE is trigonal pyramidal

The arrangement of electrons pairs/regions of electron density is tetrahedral.

You only refer to the shape of the molecule and the bond angles subtended between two atoms and the central atom. You do not refer to the arrangement of the electrons as the shape of the molecule.


That's what I meant, sorry. :colondollar:
It has the distorted tetrahedral shape so that the bond angle is different (due to lone-pair to bonded-pair repulsion> bonded-pair to bonded-pair repulsion). The structure should be called as (trigonal) pyramidal but not tetrahedral
yes, it's the same shape as ammonia. So if you include lone pairs you could say it was tetrahedral. But as you say, when naming the shape of a compound you only think about the actual bonds. So imagine the lone pair as invisible and you have an arsenic at the apex of a trigonal pyramid (not trigonal planar, by the way)
Reply 9
Avatar for chembob
chembob
10
Im sure this is exactly the same as this thread http://www.thestudentroom.co.uk/showthread.php?p=28940600&highlight=
Original post by chembob
Im sure this is exactly the same as this thread http://www.thestudentroom.co.uk/showthread.php?p=28940600&highlight=


so it is! How annoying.

Fabolous - please don't re-post exactly the same question when you've already had it answered!

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