[thuanthuan]

I have a question about distortion in bond shape of VSEPR model.

Consider a NH₃ molecule, which has 3 electron pair in covalent bond between H and N, and 1 lone pair of electron. That 4 pairs will repel one another and make the shape of a tetrahedron. Angle HNH should equal 109.5' (according to geometric calculation).
I wonder what make that angle reduce to 107' ? I think that the 3 Hs will repel one another and thus increase that NHN angle to >109.5'.

The same to H₂O molecule, angle HOH should be 109.5'. Why is it reduced to 104.5' ?

Thank you.

[AWK]

Take into accounts nonbonding electron pairs on N and O atoms

[FeLiXe]

the idea is that a nonbonding orbital takes up more space than a bonding one. that is why bonds are pushed together

[thuanthuan]

Can you explain further ? Why the two lone pairs of e^- affect the other bonding pair ? I still don't understand.
Thank you.

[FeLiXe]

all the electron pairs repel each other because negative charges repel each other

think of the sp3 hybridisation model:
in methane all the bonds are the same. we get a regular tetrahedron because then all the electron pairs have the maximum distance to each other.

in ammonia three electron pairs are pulled out by the hydrogens. the forth one is close to the N nucleus. therefore it takes up more space and pushes the others back