Question
Explain why is linear while is bent, even though both have two bonded atoms around the central atom.
Solution — Step by Step
In : Be has 2 valence electrons, both bonded to Cl. Total electron pairs around Be = 2 (both bonding). Lone pairs: 0.
In : O has 6 valence electrons, 2 bonded to H. Total electron pairs around O = 4 (2 bonding + 2 lone pairs).
VSEPR says electron pairs arrange to minimise repulsion. With 2 pairs total, they go to opposite ends — linear, . With 4 pairs total, they aim for tetrahedral geometry, .
: 2 bonding pairs, 0 lone pairs → linear (Cl–Be–Cl at ).
: 2 bonding pairs, 2 lone pairs → bent (the two O–H bonds at about , slightly less than tetrahedral because lone-pair-lone-pair repulsion compresses the bond angle).
Final answer: is linear because it has no lone pairs; is bent because of two lone pairs on oxygen.
Why This Works
VSEPR (Valence Shell Electron Pair Repulsion) classifies geometries by the total number of electron pairs around the central atom, but the molecular shape is determined by the position of the bonded atoms only. Lone pairs occupy space and push the bonds together but don’t appear in the geometric description.
Lone pair vs lone pair > lone pair vs bond pair > bond pair vs bond pair, in repulsion strength. That’s why bond angles in () are smaller than in (), which are smaller than in ().
Alternative Method
Think in terms of hybridisation: Be in uses hybridisation (linear). O in uses hybridisation; two of the four hybrid orbitals hold lone pairs, two hold bonds — tetrahedral arrangement of orbitals, bent arrangement of nuclei.
A trap: students assume “two bonds = linear.” Wrong. The total electron geometry (bonds + lone pairs) decides the angles; the molecular shape names only the atom positions.
Common Mistake
Calling “tetrahedral” because there are 4 electron pairs around oxygen. The electron-pair geometry is tetrahedral, but the molecular shape is bent (or “angular”). Always specify which one the question is asking about.