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Use electron pair repulsion theory to deduce the shape of BF3<---NH3?

i)use electron pair repulsion theory to deduce the shape of BF3<---NH3
ii)suggest the shape of the molecule if B is the central atom
ii)suggest the shape of the molecule if N is the central atom

omg, please explain how to answer these questions, I tried to draw a dot and cross diagram to get me started but I'm stuck again :confused:

help is very much appreciated

thank you

Reply 1

nevetstreblig

BF3 = Trigonal planar, 120^o
NH3 = tetrahedral. 3 bonds with a lone pair with a bond angle of 109.5

EDIT: 107, not 109.5

(edited 12 years ago)

Reply 2

SKPD11

Basically, you draw a dot and cross diagram of the outer electrons. NH3 is 107 bond angles because the Nitrogen atom has 5 electrons in the outer electron shell, froms 3 covalent bonds ( one with each hydrogen atom) but then it has a lone pair of electrons that squeezes the perfect tetrahedron from 109 to 107. Hope that helps

Reply 3

charco

Study Forum Helper

Original post by SKPD11

Basically, you draw a dot and cross diagram of the outer electrons.

This is good :smile:

but ....

Original post by SKPD11

NH3 is 107 bond angles because the Nitrogen atom has 5 electrons in the outer electron shell, froms 3 covalent bonds ( one with each hydrogen atom) but then it has a lone pair of electrons that squeezes the perfect tetrahedron from 109 to 107. Hope that helps

..... is the wrong answer.

:eek:

Reply 4

SKPD11

Original post by charco

This is good :smile:

but ....

..... is the wrong answer.

:eek:

109.5 to 107 then

Reply 5

charco

Study Forum Helper

Original post by SKPD11

109.5 to 107 then

No.

Each (central) atom has four pairs of electrons, all of which are used for bonding. You would expect each to be tetrahedral without distortion.

Reply 6

SKPD11

Original post by charco

No.

Each (central) atom has four pairs of electrons, all of which are used for bonding. You would expect each to be tetrahedral without distortion.

What ? No. 3 bonding pairs and one lone pair (NH3). Lone pair repels bonding pairs squeezing them from 109.5 to 107.

Reply 7

TattyBoJangles

Original post by SKPD11

What ? No. 3 bonding pairs and one lone pair (NH3). Lone pair repels bonding pairs squeezing them from 109.5 to 107.

This is correct. Each lone pair reduces the bond angle by ~2.5 degrees. Ammonia is therefore pyramidal with 107 degree bond angles. An ammonium ion will have 109.5 bond angles, but not ammonia.

(edited 12 years ago)

Reply 8

SKPD11

Original post by TattyBoJangles

This is correct. Each lone pair reduces the bond angle by ~2.5 degrees. Ammonia is therefore pyramidal with 107 degree bond angles. An ammonium ion will have 109.5 bond angles, but not ammonia.

Reply 9

fudgesundae

Original post by SKPD11

Nope it is tetrahedral. Same arrangement as ethane. Not exactly sure why though. Because like you said I would have thought the lone pair repels it more to distort the shape.

Reply 10

SKPD11

Original post by fudgesundae

Nope it is tetrahedral. Same arrangement as ethane. Not exactly sure why though. Because like you said I would have thought the lone pair repels it more to distort the shape.

Ohh right the original post isnt clear, I thought it was just nh3 but I guess it's some strange nh3 joining bf3. Never seen that before.

Reply 11

TattyBoJangles

Original post by fudgesundae

Nope it is tetrahedral. Same arrangement as ethane. Not exactly sure why though. Because like you said I would have thought the lone pair repels it more to distort the shape.

Ammonia is trigonal pydramidal.

Hang on, I'm confused :lol:

(post above mine)

Reply 12

fudgesundae

Original post by SKPD11

Ohh right the original post isnt clear, I thought it was just nh3 but I guess it's some strange nh3 joining bf3. Never seen that before.

Original post by TattyBoJangles

Ammonia is trigonal pydramidal.

Hang on, I'm confused :lol:

(post above mine)

Haha you two have confused me know. I'm fairly sure the OP is referring to the compound formed when BF3 and NH3 join with a dative covalent bond from the nitrogen to the Boron atom.

Reply 13

EierVonSatan

I'm sure the OP is refering to the complex :yes:

Reply 14

SKPD11

Original post by EierVonSatan

I'm sure the OP is refering to the complex :yes:

Oooo right now I can see why she might have difficulty with that!

Reply 15

MexicanKeith

BF3 on its own has only 3 pairs of electrons around the Boron so it is trigonal planar.

However, when it forms an adduct with ammonia, the product now has 4 bonding pairs around the boron (the Nitrogen lone pair becomes a bonding pair when it forms the dative bond) so in this product the geometry around Boron is tetrahedral, hope that helps :smile: