• 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 (ox) diagrams 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 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:

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

    2. 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

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

gaseous beryllium hydride BeH₂

gaseous beryllium chloride BeCl₂

carbon dioxide

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.

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120^o

electrons: two bond pairs, one lone pair

shape BENT, bond angle approximately 120^o

electrons: 3 bond pairs

shape TRIGONAL PLANAR - 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

electrons: two bond pairs and two lone pairs

BENT shape: e.g. hydrogen sulphide, H₂S, or water H₂O, i.e. H₂X with 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.

electrons: two bond pairs and two lone pairs

BENT shape: e.g. fluorine oxide (oxygen(II) fluoride) F₂O with bond angle of approximately 109^o

electrons: three bond pairs and one lone pair

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 (angle > H₂O and < CH₄).

electrons: three bond pairs and one lone pair

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⁺

electrons: 4 bond pairs

TETRAHEDRAL shape: e.g. methane CH₄, silicon hydride SiH₄ with 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, so sequence of bond angles CH₄ > NH₃ >H₂O.

electrons: 4 bond pairs

TETRAHEDRAL shape: e.g. tetrachloromethane CCl₄ or [PCl₄]⁺ with exact bond angle of 109^o

electrons: 5 bond pairs

TRIGONAL BIPYRAMID shape: e.g. phosphorus(V) fluoride (phosphorus pentafluoride) PF₅, gaseous 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.

electrons: 6 bond pairs

OCTAHEDRAL SHAPE: e.g. sulfur(VI) fluoride (sulphur hexafluoride) SF₆ or [PCl₆]^- and many transition metal complexes (see below), 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 tetrahedra, with all C-C-H and H-C-H bond angles of 109^o.

See below for more bond angle analysis of organic molecules.

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 terahedra.

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

TRANSITION METAL 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" page (under development!)

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.

      • 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

• 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 ?, 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 ? However there is evidence to suggest it oscillates between two bent conformers with bond angles 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'

• 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 

• 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  The 'scribbles' will be replaced by neat diagrams eventually!

GENERAL REVISION NOTES

GCE advanced-A2-AS-subsidiary-IB level chemistry examinations * GCE advanced-A2--AS-subsidiary-IB level chemistry examinations * GCE advanced-A2--AS-subsidiary-IB level chemistry examinations SITE PURPOSE EDUCATION - online learning or 'self-private-tuition' using revision notes, quizzes, practice tests involving ADVANCED LEVEL CHEMISTRY in the areas of REVISING only the CHEMISTRY at Doc Brown's Chemistry Clinic via HOMEPAGE in secondary school/schools, 6th form college/colleges, academy/academies or home self-study and may help with 1st year undergraduate university chemistry courses. Hopefully it will encourage interest and understanding of Chemistry, Earth Science and Radioactivity in any country of the world, though the site is written entirely in English. The website is designed to help and unofficially support students/teachers revise-learn/teach the chemistry for modular or co-ordinated examination science courses from UK QCA based AQA, OCR (Oxford and Cambridge), Nuffield, Salters, Cambridge International (CIE), London International, WJEC, CCEA etc. Also, national award assessments-examinations for BTEC-NVQ applied, additional and chemistry science courses, Advanced Subsidiary Level GCE-AS-A2-IB-KS5-BTEC-NVQ exams. National Chemistry assessment levels, International Baccalaureate, K12 higher US grade level examinations for the national curriculum for secondary schools and colleges. The notes should also provide some background theory for a coursework assignment or project. BUT please note that my on-line revision notes and quizzes are no substitute for good classroom teaching-lecturing and thorough studying of your own notes and textbooks, practicing past papers and a copy of the syllabus which are readily downloaded from the examination board sites, but I hope here and there they will lend a tutoring hand on some topic, unit, module etc. For final revision you have to be intellectually honest about what you don't know or follow, YOU have to take the stuff to pieces, analyse what you do/do not understand and reconstruct it so it all makes sense in the end. There is no other way, there are no magic secrets on how to revise and learn, its mainly down to hard work and just good old fashioned study and employing teach-yourself strategies without the need for extra tutors and tutoring lessons. I also think there is too much hit and miss revision using past papers (which I do NOT supply) and not enough systematic revision. I also hope it will help teachers in planning lessons and developing schemes of work for science-chemistry. There are no lesson plans on the site but there are plenty of quizzes to incorporate into classroom activities whether photocopied or on electronic whiteboard projector for use as self-tuition-assessment purposes and a variety of teaching and learning styles and the images may be used in Microsoft Word documents and powerpoint projections. The site seems to be used by a large number of home study tutors, particularly the revision notes. An individual tutor may print out the notes for science-chemistry learning teaching-tuition purposes and for background material for assignments and projects. I have no interest or time in producing WORD.doc or xxxx.pdf files of the notes at the moment. Neither have I time to write up many practical laboratory experiments ('lab'-'labs') at the moment, but the notes contain lots of background information of chemical reactions in terms of observations-balanced equations-reactants-products-theory etc. I also find it difficult to recommend specific exam websites or syllabus textbooks, it depends exactly on what you need, what you have time for, and there are so many of them to choose from and I do not supply past examination papers for classes. The sites resources include revision notes, quizzes and worksheets which provide support for home study or tuition for homework and coursework help e.g. science investigations for any of the key stage courses indicated, but I do not supply lesson plans.  Dr W P Brown GCE A AS A2 IB Advanced-Subsidiary Level Chemistry 10-10-2007 GCE advanced-A2--AS-subsidiary-IB level chemistry examinations * GCE advanced-A2--AS-subsidiary-IB level chemistry examinations * GCE advanced-A2--AS-subsidiary-IB level chemistry examinations