Chemistry-Shapes of molecules

What the lines mean?

A normal straight line- This line shows that bonds aren’t Pointing towards you or away from you.

A broken line-this shows a bond pointing away from you

A wedge-This shows a bond pointing towards you

Finding the number of electron pairs

To work out the shape of a molecule or an ion you need to know how many lone pairs and how many bonding pairs of electrons are on the central atom.

1) find the central atom it’s the one all the other atoms are bonded to

2) Work out how many electrons are in the outer shell of the central atom this will be the same as its group number in the periodic table

3) Add one electron for every atom that the central atom is bonded to(You can work this out from the formula of the molecule or the ion)

4) if you’re looking at ion, you need to take its charge into account. Add one electron for each negative charge or subtract one for each positive charge.

5) add up all the electrons and divide by 2 to find the number of electron pairs

6) Compare the number of electron pairs to the number of bonds to find the number of loan pairs and the number of bonding pairs on the central atom

Central atom with two electron pairs

Molecules with two electrons have a bond angle of 180° And have a linear shape. This is because the pairs of bonding electrons want to be as far away from each other as possible

Central atoms with three electron pairs

Molecules that have three electron pairs around the central atom don’t always have the same - The ship depends on the combination of bonding pairs and lone pairs of electrons.

If there are three bonding pairs of electrons, the repulsion of the charge clouds is the same between each pair and so the bond angles are all 120°. The shape of the molecule is called trigonal planar

IfIf you have a molecule with two bonding pairs of electrons and one lone pair of electrons, you’ll get a non-linear or bent molecule. The bond angle will be a little bit less than 120°

Central atom with four electron pairs

If there are four pairs of bonding electrons and no lawn pads on a central atom all the bond angles are 109.5°-The charge clouds all repel each other equally the shape of the molecule is a tetrahedral

Three bonding pairs of electrons and one lone pair

If there are three bonding pairs of electrons and one lone pair the lone pair/Bonding pair repulsion will be greater than the bonding pair/Bonding pair repulsion and so the angles between the atoms will change. This bond angle is 107°° and the shape of the molecules is trigonal bipyramidal.

Two bonding pairs of electrons and two lone pairs

If there are two bonding pairs of electrons and two lawn pair of electrons the lone pair/lone pair Repulsion will squish the bond angle even further. The bond angle will be around 104.5° and shape of the molecule is bent.

Central atoms with five electron pairs

Some central items can ‘expand the octet’.

This just means that they can have more than eight bonding electrons in the outer shell. A molecule with five bonding pairs will be trigonal bipyramidal. Repulsion Between the bonding pairs means that three of the atoms will form a trigonal planar shape with bond angles of 120° and the other two atoms will be at 90° to them

Is there a four bonding pairs and one lone pair of electron?

If there are four bonding pairs and one pair of electrons, the molecules forms a sea shape. The loan pair is always positioned where one of the trigonal planner atom would be in a trigonal bipyramidal molecule

If there are three bonding pairs and two loan pairs of electrons

If there are three bonding pairs and two loan pairs of electrons Then the molecule will be T shaped

Central Atoms with six electron pairs

A molecule with six bonding pairs will be octahedral.All of the bond angles in the molecule will be 90°

If there are five bonding pairs and one lone pair

If there are five bonding pairs and one lone pair The molecule forms a square pyramid structure. Molecules with this shape are very rare

If there are four bonding pairs and two lawn pairs of electrons

If there are four bonding pairs and two lawn pairs of electrons The molecule will be square planar

Step-by-step finding the shape of an unfamiliar molecule. Let’s go BF4^-

1) The central atom is boron

2) Boron is in group 3 so it has three electrons in its outer shell

3) The boron atom is bonded to four fluorine atoms

4) BF3 is an ion with a 1- charge So you will need to add one extra electron to account for that

5) 3+4+1 = 8 electrons in the outer shell of the central boron atom. That’s 8÷2=4 pairs

6) The Barron atom has four electron pairs around it and has made four bonds so it has four bonding pairs and no loan pairs. A molecule with four bonding pairs and no lawn pads will have a tetrahedral shape with bond angles of 109.5°

Now all you have to do is draw it

Awkward molecules

There are some special cases where molecules don’t follow the normal rules. If a molecule has multiple bonds, you treat each multiple bond as if it was one single bond when you’re working out the ship even though there’s usually slightly more propulsion between double bonds