shapes of molecules

Describe the 3D shape components

Dotted wedge- shows bond is projecting back behind the plane of the page

Wedge- shows bond coming out of the plane of the page

Straight line- bonds lie on the plane of the page

What is electron pair repulsion theory?

To work out shapes of molecules we need to look at electron pair repulsion theory

Electron pair repulsion theory states the shape of a molecule is determined by the electron pairs surrounding the central atom(only referring to outer shell e- in this case)

Theory is based on the fact that pairs of electrons repel all other electron pairs, the electron pairs now move as far apart as possible to minimise repulsion.

Note

A covalent bond is a pair of electrons

Explain what happens to electrons what is the shape of beryllium chloride

There are 2 electron pairs around the central atom

These two pairs of electrons repel each other and move as far apart as possible- the furthest distance they can move apart is in a linear structure( 180 degreees apart)

How do you determine the shape of a molecule with double or triple bonds? Use CO2 for example

We treat a multiple bond in the same way as we treat a single bond

In O=C=O these two bonding areas repel and move as far apart as possible in this case the angle between the bonding areas is 180 degree

What is the bond angle and shape of a molecule with a central atom and has 2 bonding areas WITH NO LONE PAIRS OF ELECTRONS

It will have a linear shape with a 180 degree bond angle

( bc the bonding areas repel an move as far apart as possible (180o))

What is the shape and bond angle of boron trifluoride ( no lone pairs of electrons)explain shape

The electron pairs in these 3 covalent bonds repel and move apart as far as possible - bond angle is 120 degrees and is known as a trigona planar

When do you see a trigonometry planar shape and what is its bond angle

When you have a central atom bonded with 3 pairs of electrons - as long as the central atom has no lone pairs of electrons

Bond angle is 120 degrees

Describe the shape and bond angle of methane ( CH4) no lone pairs

Methane has 4 bonding electrons around a central atom electron pairs repel each other to be as far apart as possible - forms a tetrahedral molecule with bond angle 109.5 degrees

When do you see a tetrahedral shape and what is its bond angle

When you have 4 bonding electrons around a central atom with no lone pairs of electrons. 109.5 degrees

Describe the shape and bond angle of phosphorus pentachloride (PCl5)?

Shape - trigonal bipyramidal - 5 pairs of bonding electrons around a central atom ( no lone pairs of electrons)

Electron pairs repel each other to be as far as possible/move as far apart as possible to minimise repulsion. All 5 pairs repel equally resulting in trigonal bipyramidal shape

Bond angles are 120 degrees and 90 degrees

State when we see trigonal bipyramidal shape and the Bond angle

When we have 5 pairs of bonded electrons around a central atom with no lone pairs of electrons

Bond angle 90 degrees 120 degrees

Describe the shape and bond angle of sulfur hexfluorine?

Shape- octahedral- 6 bonding pairs around the central atom with no lone pairs of electrons - electrons move as far as par as possible to minimise repulsion

Bond angle - 90 degrees

When do we see a octahedral shape and what is its bond angle

-When there is 6 bonding pairs around the central atom with no lone pairs of electrons

Bond angle - 90 degrees

What is the effect of lone pairs of electrons on the shape of molecules

Lone pairs of electrons repel more strongly tan bonding pairs, this extra repulsion decreases the bond angles of molecules by 2.5 degrees

What effect does having a dative bond do to the shape of the molecules

Having a dative bond has no effect on the shape of the ion because dative bonds behave in the same way as regular covalent bonds

Describe the shape of ammonia (NH30)and the bond angle explain the difference because it’s bonded to 3 hydrogens

Ammonia is asked on a tetrahedron. A tetrahedral has 4 bonded pairs of electrons with no lone pairs having a bond angles of 109.5degrees

But bc the lone pairs repel more strongly than bonding pairs- this extra repulsion reduces the bond angles by 2.5 degrees

Therefore the bond angle of ammonia is 107 degrees the shape of ammonia is a trigonal pyramidal

Describe the shape of the ammonium ion( NH4) and bond angle and difference between the ammonia atom

The ammonium in has a dative covalent bond ( giving a lone air of electrons to the hydrogen atom) a dative cov bond has the same level of repulsion as a regular covalent bond - meaning the bond angle in ammonium is 109.5 degrees and its a tetrahedral shape

State when you see a trigonal pyramidal shape and what the bond angle of it is ?

When you have 3 bonding pairs of electrons and 1 lone pair of electrons

Bond angle is 107 degrees

Describe the shape and bond angle of water (H2O)

Water has 4 bonding areas- so it’s based on a tetrahedral shape with bond angle 109.5 degrees.

However there are 2 lone pairs of electrons -so reduces the bond angles to 5 degrees so bond angle is 104.5 degrees

The shape V shape/ non linear

Why is the shape of water (H2O) not linear shape

Because the oxygen atom has 2 lone pairs of electrons if it didn’t have any lone pairs it would have a linear shape H-O-H

When do we see a V shape and what is its bond angle

When the molecule has 2 bonded pairs of electrons and two lone pairs of electrons

Bond angle 104.5 degrees