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(c) doc bDoc Brown's Advanced Level Chemistry for GCE AS A2 Courses

Theoretical Physical Chemistry Revision Notes

The Shapes of Molecules and Ions and bond angles related to their Electronic Structure

Part 1 from diatomic molecules to polyatomic molecules

The shapes and bond angles of a variety of molecules are described, explained and discussed using valence shell electron pair repulsion theory (VSEPR theory) and patterns of shapes deduced for 2, 3, 4, 5 and 6 groups of bonding or non-bonding electrons in the valence shell of the central atom of the molecule.  All is described and explained! i.e. how to work out the shapes of molecules and work out bond angles. Shapes can be worked out from dot & cross diagrams and bond angles deduced from the established shape. All such deductions are essentially based on electron pair(s) repulsion theory.

SHAPES OF MOLECULES INDEX: introduction * diatomic molecules * Shapes based on groups of electron pairs (bonding/non-bonding) : 2  3  4  5  6 * some more complex inorganic/organic molecules/ions * transition metal complexes * Some other molecules/ions of carbon, nitrogen, sulphur and chlorine * shapes and bond angles of organic molecules * GCSE/IGCSE/AS Science-CHEMISTRY bonding notes

 

Introduction - electron pair repulsion theory and bond angle

The shape of a molecule is determined by the number of groups of electrons around the central atom. The 'groups' might be a non-bonding single electron, a non-bonding or bonding pair of electrons, a double pair of bonding electrons or triple pair of bonding electrons etc. The electron 'groupings' repel to minimise the potential energy of the system i.e. to make the A-B-C angle as wide as possible.

The dot and cross diagrams (ox) are presented in 'Lewis style'

In the diagrams the central atom is denoted by X and attached surrounding bonded atoms by Q. The bond angle is therefore based on angle between the atoms Q-X-Q.

This is known as The VALENCE SHELL ELECTRON PAIR REPULSION THEORY MODEL (VSEPR theory, valence shell electron pair repulsion).

It has an important 'sub-rule' which affects the precise bond angle.

Any lone pairs of non-bonding electrons on the central atom X, are closer to X than bond pairs because there is no Q atom attracting/sharing the lone pair electron charge.

This will increase the repulsion between a lone pair of electrons on X and any other bonding/non-bonding on X.

The result is two-fold:

In terms of electron pair repulsion: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

As the lone pair - 'other pair' repulsion increases, the angle between these pairs increases, so the Q-X-Q angle will be slightly reduced compared to what might be expected from the 'simple' geometry of the shape (this is best illustrated by the sequence H2O, NH3 and CH4, see below)

underdeveloped test! on shapes and angles

 

Diatomic molecules

These are not considered to have a 'shape', but useful dot and cross diagram revision based on the outer valence electrons

(c) doc b H-H e.g. hydrogen H2

(c) doc b

(c) doc b

H-Cl e.g. hydrogen chloride HCl, HX in general where X = halogen

(c) doc b

(c) doc b Cl-Cl e.g. chlorine Cl2, iodine(I) chloride ICl (iodine monochloride)

(c) doc bQ and X are both halogen atoms from group 7

  O=O (c) doc boxygen molecule

  

Two groups of electrons around the central atom

two bonding pairs of electrons or two double bond pairs - linear shape - bond angle 180o

(c) doc b

(c) doc b

gaseous beryllium hydride BeH2 (Q = H, X = Be)

(c) doc b

(c) doc b

gaseous beryllium halides BeCl2 (X = Be, Q = F, Cl, X = Be)

valence bond dot and cross diagrams O=C=O

(c) doc bcarbon dioxide CO2

  [H3N-Ag-NH3]+

transition metal complex of co-ordination number 2: e.g. the diamminesilver(I) ion, [Ag(NH3)2]+, where the :NH3 ammonia molecule acts as an electron pair donor to form the bond. N-Ag-N bond angle 180o.

  

Three groups of electrons around the central atom

(c) doc b

(c) doc b

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120o

Does anyone know of any example?

(c) doc b (c) doc b

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120o

Does anyone know of an example? but for X=Q double bonds see sulphur dioxide

(c) doc b (c) doc b

electrons: 3 bond pairs

shape TRIGONAL PLANAR: Q-X-Q bond angle exactly 120o: e.g. X = B and Q = H for gaseous boron hydride BH3

(c) doc b (c) doc b

electrons: 3 bond pairs

shape TRIGONAL PLANAR: bond angle, 120o: e.g. gaseous boron trifluoride BF3

(others in the gaseous state  e.g. if Q = F or Cl then X = B or Al for F)

but for X=Q double bonds see sulfur trioxide

COCl2 (g) carbonyl dichloride (many other commonly used names! e.g. carbonyl chloride, carbon oxychloride, carbon dichloride oxide, phosgene-gas warfare agent). The dot and cross diagram shows this is another example of three groups of bonding electrons (two single C-Cl bonds and a C=O double bond) giving a trigonal planar structure with Cl-C-Cl and O=C-Cl bond angles of 120o.

     

  

Four groups of electrons around the central atom

(see also sulfate and sulfite ions)

(c) doc b

(c) doc b

electrons: two bond pairs and two lone pairs

ANGULAR or BENT shape: e.g. hydrogen sulphide, H2S, or water H2O, i.e. H2X with H-X-H bond angle of approximately 109o (actually 104.5o in water) and similarly ions like NH2-. (Q = H, X = O, S etc. in group 6)

(c) doc b(c) doc b

Why isn't the H-O-H angle 109o?  The exact H-O-H angle in H2O is 104.5o due to the extra repulsion of two lone pairs, the H-N-H is 107.5o in NH3 (one lone pair) and H-C-H is 109o (no lone pairs) because of the 'repulsion order' lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

(c) doc b (c) doc b

electrons: two bond pairs and two lone pairs

BENT shape: e.g. fluorine oxide (oxygen(II) fluoride) X = O, Q = F

F2O with bond F-O-F bond angle of approximately 109o

(c) doc b (c) doc b

electrons: three bond pairs and one lone pair

PYRAMIDAL or TRIGONAL PYRAMID shape: e.g. ammonia NH3 with bond angle of approximately 109o. Why isn't the H-N-H angle 109o? The exact H-N-H angle is 107o due to the extra repulsion of one lone pair (see below).

(c) doc b(c) doc b

Note: the exact H-O-H angle in H2O is 104.5o due to the extra repulsion of two lone pairs, the H-N-H is 107.5o in NH3 (one lone pair) and H-C-H is 109o (no lone pairs) because of the 'repulsion order' lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

(c) doc b (c) doc b

electrons: three bond pairs and one lone pair

PYRAMIDAL or TRIGONAL PYRAMID shape. e.g. nitrogen trifluoride/trichloride, NCl3, or phosphorus(III) fluoride/chloride (phosphorus trifluoride/trichloride), PF3/PCl3, with bond angles Q-X-Q of approximately 109o and similarly with ions like the oxonium ion H3O+

(Q = F, Cl etc. X = N, P etc.)

(c) doc b (c) doc b

electrons: 4 bond pairs

TETRAHEDRAL shape: e.g. methane CH4, silicon hydride SiH4 with H-X-H bond angle of 109o and similarly ions like the ammonium ion NH4+. Note: No lone pair, no extra repulsion, no reduction in angle, therefore perfect tetrahedral angle (for H-X-H angles:  CH4 > NH3 > H2O, see below).

(c) doc b(c) doc b(Q = H, X = C, Si, Ge etc. in group 4)

Note: the exact H-O-H angle in H2O is 104.5o due to the extra repulsion of two lone pairs, the H-N-H is 107.5o in NH3 (one lone pair) and H-C-H is 109o (no lone pairs) because of the 'repulsion order' lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

(c) doc b (c) doc b

electrons: 4 bond pairs

TETRAHEDRAL shape: e.g. tetrachloromethane CCl4 or [PCl4]+ with exact Cl-C-Cl and Cl-P-Cl bond angles of 109o

  

Five groups of electrons around the central atom

(c) doc b

valence bond dot and cross diagram

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electrons: 5 bond pairs

TRIGONAL BIPYRAMID shape: e.g. phosphorus(V) fluoride (phosphorus pentafluoride) PF5, gaseous phosphorus(V) chloride, PCl5, with bond angles 90o and 180o based on the vertical Q-X-Q bond and 120o based on the central trigonal planar arrangement.

Note that solid PCl5 has an ionic structure and is not a trigonal bipyramidal (bypyramid) molecule - a tetrahedral [PCl4]+ ion and an octahedral [PCl6]- ion.

  

Six groups of electrons around the central atom

(c) doc b

(c) doc b

electrons: 6 bond pairs

OCTAHEDRAL SHAPE: e.g. sulfur(VI) fluoride (sulphur hexafluoride molecule) SF6 or the [PCl6]- ion and many transition metal complexes (see below), with Q-X-Q bond angles of 90o and 180o.

  

More complex inorganic molecules/ions and organic molecules

These often are not given a particular shape name, but never-the-less, an appreciation of the 3D spatial arrangement is expected e.g.

(c) doc b

Ethane consists of two joined 'tetrahedral halves', with all C-C-H and H-C-H bond angles of 109o.

See other page for more on shape and bond angle analysis of organic molecules

 

H3N:=>BF3

would be like ethane above

Boron trifluoride (3 bonding pairs, 6 outer electrons) acts as a lone pair acceptor (Lewis acid) and ammonia (3 bond pairs) and lone pair which enables it to act as a Lewis base - a an electron pair donor. It donates the lone pair to the 4th 'vacant' boron orbital to form a sort of 'adduct' compound. Its shape is essentially the same as ethane, a sort of double tetrahedral with H-N-H, N-B-F and F-B-F bond angles of ~109o.

(c) doc b or (c) doc b

Benzene is a completely planar molecule, with all C-C-C or C-C-H bond angles of 120o.

valence bond dot and cross diagrams  

 

TRANSITION METAL COMPLEX IONS

COMPLEXES(c) doc b

The three examples below show cis/trans isomerism

(c) doc bcis/trans octahedral

(c) doc bcis/trans octahedral

(c) doc b cis/trans square planar

more details and examples on "Transition Metals" pages

All the bonds shown,__ or ...., are dative covalent, with lone electron pair donation by the ligand L, to the central metal ion

i.e. L: (c) doc b Mn+ etc.


The shapes and bond angles of BeH2 BeCl2 CO2 [Ag(NH3)2]+ BH3 BF3 BCl3 AlF3 COCl2 H2O H2S NH3 F2O PF3 PF5 PCl3 PCl5 H3O+ NCl3 CH4 CCl4 PCl4+ PCl6- SF6 H3NBF3 NH3BF3


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