Shapes of molecules.

https://www.youtube.com/watch?v=lmPiydJfGrk

What do solid lines tell us in the structure of methane / compounds

The two bonds lie on the plane of the screen, or the page

That means that these two hydrogen atoms also lie on the plane of the page

What does solid wedge tell us in structure of methane / compounds

Tells us that this bond is coming out of the plane of the page

This means that this hydrogen atom is also coming out of the plane of the page

What does dotted wedge tell us

The bond is projecting back behind the plane of the page, means hydrogen is behind the plane of the page

Electron pair repulsion theory

The shape of a molecule is determined by the electron pairs surrounding the central atom.

We are only referring to the outer shell in this case

This is based on the fact that pairs of electrons repel all of the otheer electron pairs.

The electron pairs now move as far apart as possible to minimise this repulsion.

Work out bond angles of beryllium chloride.

Cenntral atom is beryllium, this is covalently bonded to two atoms of chlorine, 2 electron pairs around the central atom

These two pairs of electrons repel each other and move as far apart as possible.

The furthest they cna move apart is in a straight line like this, its said they have a linear structure,

Angle between these two bonds is 180 degrees

What happens if a central atom has two bonding areas.

Will have linear shape with 180 degrees bond angle. Not the case if atom has lone pair of electrons

What is bond angle in boron trifluoride ( 3 bonding areas)

The bonds repel and arrange themselves towards the points of a triangle.

The bond angle between them is 120 degrees. It is called trigonal, as its based on a triangle,

If its viewed from the side, its flat, this means its planar (flat).

This shape is called trigonal planar.

What happens if molecule has 4 bonding areas

We have a tetrahedral molecule.

All bond angles in tetrahedral molecules are 109.5 degrees.

What happens if we have molecule with 6 bonding areas

The shape is called octahedral.

Bonds pointing up and down are at 90 degrees to the central plane.

The angle between bonds lying on central plane is also 90 degrees.