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 ox diagram revision based on the out 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

any example?

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120^o

any 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. gaseous boron hydride BH₃

electrons: 3 bond pairs

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

(Q = F, Cl and X = B, 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 and similarly ions like NH₂^-. Note: the exact H-O-H angle is 104.5^o due to the extra repulsion of two lone pairs. (Q = H, X = O, S etc. in group 6)

electrons: two bond pairs and two lone pairs

BENT shape: e.g. fluorine oxide (oxygen(II) fluoride)

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. Note: the exact H-N-H angle is 107^o due to the extra repulsion of one lone pair (for H-X-H angles: NH₃ > H₂O and < CH₄).

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).

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

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 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 below for more 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.

Shapes and bond angles of organic molecules

Most bond angles in organic chemistry can be accurately or approximately predicted using bond repulsion theory (with some notable exceptions at the end).

Two groups of bonding electrons around the 'middle' atom of the bond give an angle of 180^o.

Alkynes have a single bond pair and a triple bond pair around the middle carbon.

e.g. the R-C-C angle in R-CC-R alkynes, ethyne H-CC-H has linear shape

Dienes with adjacent double bonds have two double bond pairs around the 'middle' atom.

e.g. the C-C-C angle in >C=C=C< e.g. in propa-1,2-diene and buta-1,2-diene.

Diazonium salts e.g. the C-NN: bond in diazonium cations like [C₆H₅-NN:]⁺ 

Three groups of electrons around the 'middle' atom of the bond give an angle of 120^o.

Two single bond pairs and double bond pairs.

The H-C-H, C-C-H angles associated with the alkene group >C=C< which leads to a planar shape for ethene itself.

The angles associated with the carbonyl group >C=O e.g. (i) H-C=O, C-C=O in aldehydes, ketones and carboxylic acids and derivatives and (ii) C-C(=O)-O, C-C(=O)-C  in carboxylic acids and their derivatives.

(i) RCHO, RCOR and (ii) RCOOH, RCOOR, RCOCl, RCONH₂ etc.

Two 'averaged 1.5' bonds of the delocalised benzene ring and a single bond pair of the bond attaching an atom to the ring.

e.g. in benzene itself, all the -C-C-C- or -C-C-H bonds are 120^o and is a completely planar molecule.

and in benzene derivatives C-C-X where X might be Cl, N (from NO₂ or NH₂), O (from phenol -OH or ether -OR) etc.

A single bond pair, double bond pairs and a lone pair (e.g. on the N atom, x-ref ammonia).

In diazo compounds R-N=N-R, the R-N=N bond angle is 120⁰.

Four groups of electrons around the 'middle' atom of the bond usually gives an angle of about 109^o. The orbitals would point to the corners of a tetrahedron.

Four single bond pairs give all the C-C-C or H-C-H or C-C-H angles in most (see below) saturated systems e.g. alkanes, chloroalkanes etc..

Also the R-N-R angle in the quaternary ammonium salt ion [NR₄]⁺.

Three single bond pairs and a lone pair (x-ref ammonia above).

e.g. the H-N-H, C-N-H bond angles in amines R-NH₂, R-NH-R, :NR₃ etc.

Two single bond pairs and two non-bonding lone pairs (x-ref water above).

e.g. the C-O-H angle in alcohols and phenols and the >C-O-C< angle in ethers and esters

Some significant exceptions to the above general rules.

Despite the four single bond pairs, the C-C-C bond angle in cyclopropane is a 'forced' 60^o. and the H-C-H angle is over 109⁰, from a distorted 'tetrahedral' situation.

Despite the four single bond pairs, the C-C-C bond angle in cyclobutane is on face value a forced 90^o and the H-C-H angle is over 109⁰. However there is evidence to suggest it oscillates between two bent conformers with an 'average' bond angle of ~109^o.

For cyclopentane onwards, the ring is 'puckered' with C-C-C, C-C-H and H-C-H bond angles of ~109^o.

EXAMPLES analysed and summarised for you ...

but I suggest you sketch some out and mark on all the angles, see the end 'scribbles'

methanol all H-C-H, C-O-H and H-C-O angles ~109^o

chloromethane all H-C-H and H-C-Cl angles ~109^o tetrahedral shape

propane all H-C-H, C-C-H or C-C-C angles are 109^o 

ethene

all H-C-H or H-C=C angles are 120^o in the completely planar molecule of ethene

propene H-C-H 109^o in the CH₃- group, but the

H-C=C, C-C=C, C=C-H and =CH₂ angles are 120^o

propyne the H-CC and CC-C angles are 180^o

and the C-C-H and H-C-H in -CH₃ are 109^o linear molecule shape

methylbenzene C-C-C in ring 120^o and C-C-C(H₃) off the ring 120^o

but the C-C-H of the C-CH₃ and the H-C-H in the -CH₃ off the ring are 109^o 

bromoethaneall H-C-H, H-C-C, C-C-Br, H-C-Br angles are 109^o 

ethanolall H-C-H, H-C-C, C-C-O, H-C-O, C-O-H angles are 109^o 

methoxymethaneall H-C-H, H-C-O, C-O-C angles are all 109^o 

phenolC-C-C in ring 120, C-C-H of ring 120^o and C-O-H off ring is 109^o 

ethylamine all H-C-H, C-C-H, C-C-N and C-N-H angles are all 109^o 

butanone H-C-C, H-C-H and (O=)C-C-C on right are all 109^o 

and C-C=O on left, C-C(=O)-C and O=C-C on right are 120^o 

methanoic acidH-C=O, H-C-O(-H) and O=C-O are 120^o and C-O-H is 109^o 

ethanamide   H-C-H, H-C-C, H-N-H and C-N-H are all 109^o 

and C-C=O, C-C-N and O=C-N are 120^o 

ethyl ethanoate   all H-C-H, O-C-C (right), H-C-C (left)

and C-C-H (right) are all 109^o, and C-C=O, C-C-O (left) and O=C-O are 120^o 

ethanoyl chloride   H-C-H and H-C-C are 109^o 

and C-C=O, C-C-Cl and O=C-Cl are all 120^o 

diazo dye all the C-C-C, C-C-H, C-N=N, C-C-N(=) bond angles

of/off the ring are all 120^o but the C-O-H of the phenol group on the right is 109^o.

the SCRIBBLES!

which will eventually be replaced by neater diagrams!

SEE ALSO Appendix 1-4 on separate page: The shapes, with ox diagrams and bond angles, of some other molecules/ions of carbon, nitrogen, sulphur and chlorine besides those on this page and again the 'scribbles' will be replaced by neat diagrams eventually!

Copyright Dr W P Brown 2000-2010 All rights reserved on the revision notes pages, quizzes, worksheets, x-words etc.