Question

CCl4 - Tetrahedral SiO2 - linear NH3 - Trigonal Pyramidal H20 - Tetrahedral?

Why am i not getting this? I drew out the lewis structures but i'm still not getting it right

Answer

Looks like you have it about right.

H2O is bent or V-shaped.

You might be a little confused about the difference between orbital geometry and molecular geometry.

The central atoms in CCl4, NH3, and H2O all have an sp3 orbital geometry.

The central atom is surrounded by 4 electron pairs.

That's central to defining the shape of the molecule.

But molecular geometry just takes into account the bonded orbitals.

CCl4 has 4 bonded orbitals so it's tetrahedral.

NH3 only has 3 bonded orbitals and 1 unshared pair of electrons.

The N is at the top of the pyramid, not another atom like in CCl4.

H2O only has 2 bonded orbitals and 2 unshared pairs of electrons.

Since the bond angle is based on a tetrahedron (4 pairs of electrons,) the molecule can't be linear.

Since there are only 2 bonds, the molecule can't be tetrahedral. Thus the V-shape.

The dot diagram for SiO2 probably isn't the best clue to its geometry.

It forms a network crystal rather than a molecule.

http://en.wikipedia.org/wiki/Silicon_dio…

shows a tetrahedrally based crystal.

Each Si is surrounded by 4 O's arranged tetrahedrally.

Each O is attached to 2 Si's, so the simplest ratio winds up SiO2.

Predicting that a single SiO2 molecule would be linear is a good guess

based on an analog with CO2.

The dot diagram shows two double bonds.

Each double bond requires a sigma and a pi bond.

You can think of Si as having an 1 s- and 3 p-orbitals.

Each pi bond requires a p-orbital,

leaving an s-orbital and a p-orbital to be hybridized.

sp hybrids are 180degrees apart so the molecule is linear.

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