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

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

Q and X are both halogen atoms from group 7

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

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

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

carbon dioxide CO₂

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 180^o.

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120^o

Does anyone know of any example?

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120^o

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

electrons: 3 bond pairs

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

electrons: 3 bond pairs

shape TRIGONAL PLANAR: bond angle, 120^o: 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 120^o.

(see also sulfate and sulfite ions)

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 109^o (actually 104.5^o in water) and similarly ions like NH₂^-. (Q = H, X = O, S etc. in group 6)

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

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 109^o

electrons: three bond pairs and one lone pair

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

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

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 109^o and similarly with ions like the oxonium ion H₃O⁺

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

electrons: 4 bond pairs

TETRAHEDRAL shape: e.g. methane CH₄, silicon hydride SiH₄ with H-X-H bond angle of 109^o 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).

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

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

electrons: 4 bond pairs

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

valence bond dot and cross diagram

electrons: 5 bond pairs

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

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 90^o and 180^o.

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

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

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

_{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 ~109^o.

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

TRANSITION METAL COMPLEX IONS

COMPLEXES

The three examples below show cis/trans isomerism

cis/trans octahedral

cis/trans octahedral

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: 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

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

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