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Syllabus
Syllabus
What does this mean?
What does this mean?
VSEPR Theory
VSEPR Theory
We (should) already know how to draw a Lewis Structure for simple molecules like Ammonia (left).
We can extend this to simple polyatomic ions
So, Ammonium ions (NH4+) form when Ammonia (NH3) gains a Hydrogen ion (H+).
The Lewis Structure of Ammonium is shown, right.
Note, the charge is spread across all 5 atoms and this is indicated by the [ ].
Shapes of Molecules and VSEPR
Shapes of Molecules and VSEPR
But even these don’t give any information about the shape of the molecule.
VSEPR – stands for Valence Shell Electron-Pair Repulsion theory and states that the shape of a molecule (about its central atom) is entirely due to the repulsions between pairs of electrons in the outside shell of the central atom.
It assumes that:
· The molecule will take up the shape of minimum repulsion.
· Lone pairs repel more than bond pairs
· Double/Treble bond do not repel more than single bonds
2 Electron Domains.
2 Electron Domains.
The IB call lone-pairs, single & double bonds electron domains.
The furthest apart two domains can be is 180o a straight line or Linear Structure.
Example – CO2 ,right.
Note: the pairs of electrons on the Oxygen atoms are ignored.
3 Electron Domains.
3 Electron Domains.
The furthest apart three domains can be is 120o.
This makes a Trigonal planar structure if all domains are bonds, or a bent (v-shape) molecule if one of the domains is a lone pair.
Bond Angles: Trigonal Planar= 120o, Bent = 117-118o due to lone pair since each lone pair removes 2 to 2.5o.
4 Electron Domains.
The furthest apart four domains can be is 109.5o.
Not obvious, just learn it!
This makes a Tetrahedral structure if all domains are bonds.
Or a trigonal pyramid if one of the domains is a lone pair.
Or a different Bent shape if two are lone pairs.
So, the trigonal pyramid bond angle will be around 107.
And the bent angle will be around 104.5o since each lone pair removes 2 to 2.5o.
VIDEO
VIDEO
Questions
Questions
Draw Dot-Cross diagrams (Lewis Structures) for the following molecules and predict the bond angles.
1. SiCl4
2. BF3
3. NH3
4. NH4+
5. H2S
6. BeCl2
Resonance Structures.
Resonance Structures.
If you draw a dot-cross diagram for the Nitrate ion (NO3-) it’s not possible for all the Oxygen atoms to bond equally, although it is possible for all 4 atoms to have a full Octet providing one bond is made as a coordinate (dative bond).
But there’s no reason for any one Oxygen atom to behave differently from any other so the molecule flips between all three possible structures – it exists in resonance between all three.
VIDEO
VIDEO
Questions
Questions
Try to draw a dot-cross diagram for the Nitrite ion (NO2-)
How will it resonate?
Try to draw a dot-cross diagram for the Sulphate ion (SO42-) – it may take several attempts!
How will it resonate?
Try to draw a dot-cross diagram for the Sulphite ion (SO32-)
How will it resonate?
Molecular Polarity
Molecular Polarity
We’ve seen how to find bond polarity, but polar bonds don’t always make polar molecules.
If there’s no ∂+ and no ∂- end to the molecule it isn’t polar.
In CCl4 all the dipoles cancel = non-polar
In HCCl3 they don’t = polar
Questions.
Questions.
Draw the shapes of the following molecules and decide which is polar.
1. NH3
2. BF3
3. SiCl4
4. SF2
5. H2O
6. SO3 (resonance!)
Giant Covalent Structures
Giant Covalent Structures
Most covalent substances exist as small molecules.
Some exist in giant lattice structures where every atom is covalently bonded to its neighbours.
You should know the structures of:
1. Diamond - tetrahedral Carbon
2. Silicon Dioxide (silica) /sand
Also tetrahedral but with an O atom between each Si
3. Graphite - also Carbon but in hexagonally bonded sheets weakly held to each other by London Forces (see 4.4 – Intermolecular Forces).
Properties
Properties
1 Melting Points – Silica, Diamond and Graphite all have High Tm
Why?
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2 Conductivity – Silica and Diamond are electrical insulators.
Why?
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3 Conductivity – Graphite is an electrical conductor.
Why?
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4 Hardness –Diamond is very hard, Graphite is soft
Why?
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Other Allotropes of Carbon
Other Allotropes of Carbon
Fullerenes
Fullerenes
Apart from Diamond and Graphite, Carbon can exist as Fullerenes, the best known of which is Buckminsterfullerene (Buckyballs)
Questions.
Questions.
1. How many Carbon atoms in a Buckyball?
2. Is this a molecular structure or a Giant Covalent Structure?
Graphene
Graphene
Single layers of Graphite are called Graphene
Buckyballs can be extended into Nano-tubes – what uses might they be put to?
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VIDEO
VIDEO
Co-ordinate (Dative) bonds
Co-ordinate (Dative) bonds
Covalent Bonds in which one atom donates both electrons to the bond pair are called co-ordinate bonds.
They are every bit as strong as the equivalent “ordinary” covalent bond and so can’t be distinguished from them.
But we indicate them as arrows (à) pointing from the donating atom to the acceptor when drawing displayed formulae.
Questions
Draw the dot-cross diagram and a displayed formula of:
1. H3O+ - Hydronium ion
2. NH4+ -Ammonium ion
3. Carbon Monoxide
4. Dimerised Aluminium Chloride (Al2Cl6)
Structure and properties.
Structure and properties.
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