Jall Barke
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I'm kinda confused by this question.

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1 (e) Sodium reacts with ammonia to form the compound NaNH2 which contains the NH2– ion.  Draw the shape of the NH2–
ion, including any lone pairs of electrons.
Name the shape made by the three atoms in the NH2– ion.
–
Now the way I worked it was N usually has 5 e- in the outer shell, but it's a negatively charged ion so it has 6 e- this time round. Then, since it's bonded to 2 hydrogens I took off 2, left with 4. So that leaves 4 e- for lone pairs so I have 2 lone pairs. I drew my H in the center, lone pair up top, and on the bottom half I drew a lone pair and two hydrogens and named it as it's shape, tetrahedral.

In the mark scheme it says specifically 'not tetrahedral'. Does anybody know where I'm going wrong?
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TiTo20
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(Original post by Jall Barke)
I'm kinda confused by this question.

Code:
1 (e) Sodium reacts with ammonia to form the compound NaNH2 which contains the NH2– ion.  Draw the shape of the NH2–
ion, including any lone pairs of electrons.
Name the shape made by the three atoms in the NH2– ion.
–
Now the way I worked it was N usually has 5 e- in the outer shell, but it's a negatively charged ion so it has 6 e- this time round. Then, since it's bonded to 2 hydrogens I took off 2, left with 4. So that leaves 4 e- for lone pairs so I have 2 lone pairs. I drew my H in the center, lone pair up top, and on the bottom half I drew a lone pair and two hydrogens and named it as it's shape, tetrahedral.

In the mark scheme it says specifically 'not tetrahedral'. Does anybody know where I'm going wrong?
You forget that the lone pairs will want to repel each other, so the molecule becomes like water - a bent shape. Is that what the mark scheme says?


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charco
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(Original post by Jall Barke)
I'm kinda confused by this question.

Code:
1 (e) Sodium reacts with ammonia to form the compound NaNH2 which contains the NH2– ion.  Draw the shape of the NH2–
ion, including any lone pairs of electrons.
Name the shape made by the three atoms in the NH2– ion.
–
Now the way I worked it was N usually has 5 e- in the outer shell, but it's a negatively charged ion so it has 6 e- this time round. Then, since it's bonded to 2 hydrogens I took off 2, left with 4. So that leaves 4 e- for lone pairs so I have 2 lone pairs. I drew my H in the center, lone pair up top, and on the bottom half I drew a lone pair and two hydrogens and named it as it's shape, tetrahedral.

In the mark scheme it says specifically 'not tetrahedral'. Does anybody know where I'm going wrong?
The previous poster is correct, but more to the point you MUST remember that electrons are invisible when it comes to molecular (or ionic) shape.

The structure is based on a tetrahedral arrangement of electrons, BUT two of the electron pairs are invisible leaving only the H-N-H, which is angular or bent in shape.

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gauts
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If it has 2 Bond pairs and 2 lone pairs it is a bent structure with 104.5 degrees bond angle
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Infraspecies
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NH2-

From VSEPR;

N contributes 5 valence electrons
2 N-H bonds contribute 2 electrons
-ve charge contributes 1 electron
Hence Total is 8 electrons

The Lewis structure of N gives 2 2 electron bonding regions, hence there are 2 lone pairs

4 pairs of electrons; based on the tetrahedral ideal geometry

Hence, the NH2- molecules will be planar, and based on tetrahedral geometry. Called "bent", the bond angle is less than 109.5 degrees.
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charco
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(Original post by Infraspecies)
NH2-

From VSEPR;

N contributes 5 valence electrons
2 N-H bonds contribute 2 electrons
-ve charge contributes 1 electron
Hence Total is 8 electrons

The Lewis structure of N gives 2 2 electron bonding regions, hence there are 2 lone pairs

4 pairs of electrons; based on the tetrahedral ideal geometry

Hence, the NH2- molecules will be planar, and based on tetrahedral geometry. Called "bent", the bond angle is less than 109.5 degrees.
Logically, it is impossible to have a two or three atom system which is NOT planar!!!!
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Infraspecies
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(Original post by charco)
Logically, it is impossible to have a two or three atom system which is NOT planar!!!!
Yes, I know. Anything else you want to contribute?
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charco
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(Original post by Infraspecies)
Yes, I know. Anything else you want to contribute?
Don't think so ... I'll let you know.
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Infraspecies
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(Original post by charco)
Don't think so ... I'll let you know.
You can find me on the edge of my seat.
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ramadan123456
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Mate your wrong. It would have been a tetrahedral but has two lone pairs. As lone pairs repel more than bonded pairs it means that the shape is non linear an has a difference of 104.5 degrees between them.

OCRA Chemistry A level examiner.

PS: If you want the mark schemes for up and coming alevel exams I have them.
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Pigster
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(Original post by ramadan123456)
Mate your wrong. It would have been a tetrahedral but has two lone pairs. As lone pairs repel more than bonded pairs it means that the shape is non linear an has a difference of 104.5 degrees between them.

OCRA Chemistry A level examiner.

PS: If you want the mark schemes for up and coming alevel exams I have them.
Who are you saying is wrong?
What was said that you disagree with?

Do you have the mark schemes for the 2020 papers?
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ramadan123456
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(Original post by Pigster)
Who are you saying is wrong?
What was said that you disagree with?

Do you have the mark schemes for the 2020 papers?
Yes i do actually
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Pigster
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(Original post by ramadan123456)
Yes i do actually
I'm still unsure who you were disagreeing with when you wrote "Mate your wrong". Or in fact what anyone said that was wrong.

I am also surprised that anyone will access to the 2020 papers would be claiming that on TSR.
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ramadan123456
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I am disagreeing to everyone is opinion hear. I did a degree in chemistry and mastered VSEPR theory years ago. Im right yourn wrong. Geddit.
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