Discovering Molecular Shapes

An Introduction to Valence Shell Electron Repulsion Techniques

Systems Tend toward Low Energy States

This high potential energy state is “unstable.”

What will happen if the triangular block is removed?

Systems Tend toward Low Energy States

See what happens if the triangular block is removed?

Systems Tend toward Low Energy StatesHigh potential energy state

Low potential energy state

Unstable high energy state tends to change to low energy, and give off light, heat or other energy

Now oppositely charged particles attract

Attractive Force

Separate, these charges are at high potential energy

They will reduce energy by coming together

Identically charged particles repel

Repulsive Force

Together, these charges are at high potential energy

They will reduce energy by separating

Identically charged particles repel

Repulsive Force

They will reduce energy by separating

Identically charged particles repel

They will reduce energy by separating

Now we can discuss how electron pairs arrange

themselves around the nucleus

What do you think controls their position?

They will take the positions that produce the lowest

potential energy

Remember, the electrons in their orbitals are attracted to the

nucleus.That means they can

swing around the nucleus like spokes on a pivot

Remember, the electrons in their orbitals are attracted to the

nucleus.

Electrons attracted to the nucleus may move around the nucleus, but will always stay

close to it

Let's look at how two electron pairs would swing around the nucleus

If we position them at 90º angles, they swing away

from each other

Let's look at how two electron pairs would swing around the nucleus

Does it make sense that they would

get as far away from

each other as possible?

Two electron centers would be at lowest energy at a 180 degree angle

So they would

vibrate about an angle of

180º

Let's experiment with two, three and four electron pairs

The purpose of the investigation is to find out what angles to expect the

electron pairs to make with each other.

Let's experiment with two, three and four electron pairs

Then we can attach atoms to the orbitals at those angles and see what the molecular geometry will be.