why on earth is this

Okay so apparently (according to an edexcel exam paper) SF₆ is an octahedral shaped molecules with bond angles of 90degrees and 180degrees

BUT, in my text book it makes a point of saying that octahedral molecules "have all bond angles of 90degrees" :mad:

Why is this?

Reply 1

username913907

Exam boards are ****. It's a totally symmetric molecule so all bonds are equivalent at 90 degrees separation. Of course the angle to the Cl opposite is 180 but that is never usually brought up.

Reply 2

upthegunners

Original post by JMaydom

Exam boards are ****. It's a totally symmetric molecule so all bonds are equivalent at 90 degrees separation. Of course the angle to the Cl opposite is 180 but that is never usually brought up.

Edexcel is such a scummy exam board. The question is:
"Which statement best describes the shape and bondangles in the molecule SF_6?
A: Octahedral, 90d and 180d
B:Trigonal bipyramidal, 90d and 180d
C:Octahedral,90d and 120d
D:Trigonal bipyramidal 90d and 120d"

Okay so i obviously scored out B and D immediately but how on earth am i ever meant to prepare for such a question when no website, textbook or any other resource for that matter has that answer

piece of **** exam board can burn in hell

Reply 3

krisshP

C = answer just like ^ post

Reply 4

upthegunners

Original post by krisshP

C = answer just like ^ post

why though? Apparently octahedral molecules only have bond angles of 90degrees

Reply 5

AtomicMan

Original post by upthegunners

why though? Apparently octahedral molecules only have bond angles of 90degrees

I think the 180 degrees refers to the bond angle between the flourine at the top of the octahedran and the flourine at the bottom of the octahedran

Reply 6

upthegunners

Original post by AtomicMan

I think the 180 degrees refers to the bond angle between the flourine at the top of the octahedran and the flourine at the bottom of the octahedran

then why is the answer c? :lol:

Reply 7

AtomicMan

Original post by upthegunners

then why is the answer c? :lol:

I have no clue, 90 degrees and 120 degrees describes trigonal bypramidal :s-smilie:

Reply 8

upthegunners

Original post by AtomicMan

I have no clue, 90 degrees and 120 degrees describes trigonal bypramidal :s-smilie:

Well that's in an edexcel unit 2 pastpaper..

Reply 9

Albino

Original post by upthegunners

Well that's in an edexcel unit 2 pastpaper..

The answer should be 90 180, edexcel really don't have a clue about much

Reply 10

krisshP

Original post by upthegunners

why though? Apparently octahedral molecules only have bond angles of 90degrees

Oops! Sorry. I thought you were referring to a molecule with a traingular bipyramidal shape.

SF6 really has a 90* bond angle, see
http://www.chm.bris.ac.uk/motm/SF6/sf6-sf.gif
http://4.bp.blogspot.com/_YyFkK2rbzYw/SPsJTFlmNFI/AAAAAAAAABg/TMHf1N8lBlA/s320/octa.jpg

Edexcel are dodgy in Chemistry then I guess.

(edited 11 years ago)

Reply 11

username913907

Original post by krisshP

C = answer just like ^ post

Ummm, NOOOO! the answer would be 90d and 180d. Why it isn't just 90d only though.

Reply 12

krisshP

Original post by JMaydom

Ummm, NOOOO! the answer would be 90d and 180d. Why it isn't just 90d only though.

WHY 180* ? I understand why it is 90*.

Reply 13

charco

Study Forum Helper

Original post by krisshP

WHY 180* ? I understand why it is 90*.

They are being pedantic. The bond angle in carbon dioxide is described as 180º with the atoms all being linear.

You could argue that you can find groups of three atoms lying in a linear arrangement within the octahedral structure, eg sulphur with two sets of equatorial fluorines, and one set of axial fluorines. These would have a bond angle of 180º.

However, if you take pendantry to extremes, you could also argue for the presence of 270º bond angles in sulphur hexafluoride (the reflex angles).

Like many such issues, just learn what the exam board require and use it.

Reply 14

Midlander

Um, it's 90 and 180. The cis bond angles (around the square) are 90 and the trans bond angle (both at opposite ends of the octahedron and diagonally opposite sides of the square) is 180.

Think of it as the combination of linear and square planar geometries.

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