FXe2 Bond Angle: 104.5° or 90°?

[Tanya Back]

Hey Guys!

i was wondering what would be the bond angle of FXe2..would it be 104.5 or 90?

thanx
Tanya

 

[chem_tr]

Hello, I think the correct formula should be $\displaystyle XeF_2$, am I correct?

You can use VSEPR principles to find the correct geometry for this compound; just consider the electronic configurations of the two atoms with covalent bonding, not ionic one. Use 54 electrons for xenon and 9 electrons for fluorine; if they are bonded each other via covalent bonds, then one of them must come from Xe, and the other from F.

1. Write the electronic configurations.
2. Every atom except hydrogen obeys octet rule; I mean, the outer shell has 8 electrons. So in $\displaystyle XeF_2$, Xe+2F must have 3*8=24 total electrons.
3. How many electrons are available? I think there is a shortage of electrons, and this number must be provided by mutual contributions of these three atoms via covalent bonding.
4. Write the electropositive atom (here, xenon) in the central position, and surround it with two electronegative ones.
5. Determine the bond order; here it is 1.
6. Determine how many n electrons are possible; here Xe has to have some non-bonding electrons.
7. As the central atom has some non-bonding electrons, VSEPR theory suggests that there must be a deviation of geometry. Just do the calculations and give feedback. I presume the angle will be 90°.

 

[maverick280857]

There is another way out too. If you remember the basic list of geometries (by experience of course) then you can use the following trick. Compute a quantity called the steric number which equals the sum of the number of sigma bonds and the number of lone pairs. Then the steric number can be related to hybridization and geometry. For XeF2, the Xenon atom has 8 outer electrons of which 2 are spent in bonding to flourine so effectively you have 3 lone pairs and 2 sigma bonds...means steric number = 5 so the hybridization is $$sp^3d$$ and the electron geometry is trigonal bipyramidal while the molecular geometry is linear (thats the only energetically favorable way you can arrange 3 lone pairs and 2 atoms in space). The bond angle is therefore 90 degrees (some people refer to the Xe-F-Xe angle, which is 180 degrees).

Please note that this "trick" is not to be used without discretion and I mentioned it purely because it is faster. If you're a newbie you MUST follow chem_tr's advice and use this perhaps on tests or when you're really in a hurry and cannot afford to find things out on paper.

Cheers
vivek