Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding Carbon)

Includes potential questions, shapes of molecules, and hints/basics. Does not include carbon allotropes section

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  • Created on: 28-05-14 09:38
Preview of Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding Carbon)

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Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding
Carbon)
Created using a combination of Edexcel textbook, CGP revision guide, George Facer Unit Guide, and
Edexcel revision guide.
They will ask you to draw the shape, state the bond angle(s), name the type of shape for the
most basic questions. You will need to also be able to compare bond angles, shapes, and
explain why they are different.
*These are slightly more difficult questions that may come up in Sections B/C.
Q: What shape and how many lone pairs on H2S?
H2S shape ­ bent, two lp. Has 6 valence electrons, 2 are used to form two single bonds with
hydrogen, leaving four electrons, thus two bonding pairs.
REMEMBER: Explain how you figured out the number of lp's by stating number of valence
electrons, bp's AND lp's
Q: Explain how electron pair repulsion theory predicts the shape of the planar SO3 molecule.
According to VSEPRT, the strength of repulsion between types of electrons is as follows:
lp/lp>lp/bp>bp/bp. As the SO3 molecule contains three bonding pairs only, they will distance
themselves equally around the central atom to minimise repulsion, and the best way is to arrange by
the corners of an equilateral triangle, creating the trigonal planar shape and bond angle of 120.
(NB: The shapes of molecules is predicted from the number of electrons around the central atom)
Q: Why ICl4- is not tetrahedral?
It is square planar because there are both four chlorine atoms, and a further two lp's of electrons,
and according to VSEPRT, lp/lp has the greatest repulsion, so will be distanced furthest apart, hence
on opposite sides of the iodine (both above and below), decreasing the bond angle between
chlorine atoms due to them having the lesser repulsion of bp/bp.
Q: What does displayed formula wrongly suggest about alkane structures?
That the shape around each carbon atom is square planar and the molecule is planar overall, which is
incorrect as each carbon central molecule has a tetrahedral structure. It also suggests the bond angle
between carbon atoms is 180 degrees, where it is in fact 109.5 due to the tetrahedral shape around
the carbon atom to minimise repulsion.
Q: Bond angles around a double bond?
Trigonal planar shape with bond angles approximately 120 due to lack of rotation from the
delocalised pi electrons.
Q: (both the same style of Q) i- Why shapes of NCl3 and BCl3 are different.
ii- Why HNH bond angle in NH3 is less than FBF in BF3
i- BCl3 has 3 electron pairs around the central B atom; NCl3 has four electron pairs around the central
NH3 atom, including 1 lone pair, hence less space, creating different bond angles.

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Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding
Carbon)
ii- 3 electron pairs around central B atom, but 4 electron pairs around central N atom, hence less
space, as the ammonia molecule has an extra pair of electrons around the central N atom.
NB: Work out the number of electron pairs (regardless of type), and if they are different, use this as
the answer.…read more

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Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding
Carbon)
Names of molecules and drawing the structures
* Linear
eg. CO2, BeCl2, HCl
O=C=O
Cl-Be-Cl Bond angle of 180
H-Cl
* Bent ­ usually caused by lone pairs on central atom of a `linear' shape
eg. H2O, SO2
Bond angle of 109.5, as explained in Q's Bond angle=104 as two lp's.
*Trigonal planar ­ equal distribution of 3 atoms around a central atom (not if it's 2 atoms and lp for
example)
eg.…read more

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Shapes of Molecules topic. Edexcel. Unit 2. AS Chemistry. (excluding
Carbon)
*Square planar
eg. XeF4, ICl4-
BA=90 BA=90
*Trigonal bipyramidal
eg. PCl5
BA = 90 and 120
*Octahedral
eg. SF6
BA = 90
*Pentagonal bipyramidal
eg.…read more

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