[funboy]

The question is

Explain why BeF2 is linear and SF2 angular

Be has 2 electrons available for bonding
F has 7 electrons available for bonding

therefore

Be--F--Be
This is linear

With SF2

S has 6 electrons available for bonding
F has 7 electrons available for bonding

when I draw the lewis dot structure for this atom I have two lone pairs of electrons at opposite sides of the molecule, wouldnt the repulsion of these two lone pairs (that have the same repulsion) push this molecule into a linear molecule??

I am not overly familiar with drawing dot structures, but I would see
   ..
 F-S-F
   ..

Something like that.

I want to say that if I look at the level which the bond takes place and the levels of the two lone pairs there would be a different amount of repulsion, but Im not possitive.

Im guessing that one lone pair is from the 3p level and the other is from the 3s level.

Am I close??

BTW

Im loving chemistry now.

[xiankai]

Im guessing that one lone pair is from the 3p level and the other is from the 3s level.

what actually takes place is hybridisation, so that all the bonds are equal. you may like to look it up further.

also, when considering the shape of the molecule; only bonds about the central atom need to be considered. this may help you determine the shape faster

[Borek]

Be--F--Be

Check this out

Im guessing that one lone pair is from the 3p level and the other is from the 3s level.

See xiankai suggestion.

[Donaldson Tan]

what actually takes place is hybridisation, so that all the bonds are equal.

Google for Valence Shell Electron Pair Repulsion (VSEPR) theory

[Korokian]

http://web.centre.edu/anesk/che117/sept21.ppt

theres some vsepr geometry help for you

[mike]

Try the student FAQ:
http://www.chemicalforums.com/index.php?board=4;action=display;threadid=5801;start=msg25789#msg25789

[english]

Beryllium fluoride is linear because Beryllium only needs 4 electrons to attain an octet.  Boron only needs 6.  These are two exceptions to the rules.  Born and Beryllium are electron deficient atoms that can form several gaseous compounds, such as BeF₂.

So Beryllium has no lone pairs and two bonds.  Therefore it is sp hybridized and takes a linear molecular geometry.

Note— Number of bonds = number of hybrid orbitals mixed, so in the case of beryllium fluoride we have two bonds and no lone pairs, making sp the only viable hybridization.

[Donaldson Tan]

Beryllium fluoride is linear because Beryllium only needs 4 electrons to attain an octet.

Even after bonding, the Beryllium atom does not contain 8 electrons in its valence shell. The term octet does not apply. In total, there are only 2 electron pairs in the valence shell of Beryllium in BeF₂. By VSEPR Theory, the hybridisation of the Beryllium therefore must be sp, which indicates a bond angle of 180^o.

Born and Beryllium are electron deficient atoms that can form several gaseous compounds, such as BeF₂.

BeF₂ has a melting point of 552^oC. However, compounds with electron deficient atoms tend to dimerise to achieve octet, such as AlCl₃.

Note— Number of bonds = number of hybrid orbitals mixed

This is so not true. There 4 electron pairs in the valence shell of S in SF₂ - 2 bonding pairs and 2 lone pairs/ By VSEPR Theory, the hybridisation of S must be sp³, which indicates a bond angle of 109.5^o. However, due to uneven distribution of repulsion between the different type of electron pairs, the bond angle is actually smaller. This is because electronic repulsion from the lone pairs pushes the 2 bonding pairs closer to each other, thus reducing the bond angle.

[english]

This is so not true. There 4 electron pairs in the valence shell of S in SF2 - 2 bonding pairs and 2 lone pairs/ By VSEPR Theory, the hybridisation of S must be sp3, which indicates a bond angle of 109.5°. However, due to uneven distribution of repulsion between the different type of electron pairs, the bond angle is actually smaller. This is because electronic repulsion from the lone pairs pushes the 2 bonding pairs closer to each other, thus reducing the bond angle.

I know.

I tend to think of lone pairs as sigma bonds, so I have a habit of saying number of bonds and not saying number of bonds plus lone pairs. 

But I was only talking about BeF₂ anyway.