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<meta name="description" content="For some reason students seem to find it very difficult to predict the shape and bond angles of a simple molecule or ion using Valence Shell Electron Pair Repulsion Theory (VSEPR). At first sight this seems strange because the theory really is very simple. Essentially there are only three basic ideas:Applying the first idea leads to the fact that there are only five basic shapes to be considered. In fact there are only...">
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science </a></li><li><a href="../19519/metals-and-icp-spectroscopy-1.html">Metals and ICP spectroscopy</a></li><li><a href="../19541/catalysts--1.html">Catalysts </a></li><li><a href="../19580/liquid-crystals-1.html">Liquid crystals</a></li><li><a href="../19665/polymers--1.html">Polymers </a></li><li><a href="../19672/nanotechnology-1.html">Nanotechnology</a></li><li><a href="../19843/environmental-impact-plastics-1.html">Environmental impact - plastics</a></li><li><a href="../19858/superconducting-metals-x-ray-crystallography--1.html">Superconducting metals & X-ray crystallography </a></li><li><a href="../19666/condensation-polymers-1.html">Condensation polymers</a></li><li><a href="../19916/environmental-impact-heavy-metals-1.html">Environmental impact - heavy metals</a></li></ul></li><li><a href="../19284/option-b--1.html" class="father">Option B </a><ul class="level-2"><li><a href="../19290/introduction-to-biochemistry--1.html">Introduction to biochemistry </a></li><li><a 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nuclear fission reactions</a></li><li><a href="../19429/solar-energy-1.html">Solar energy</a></li><li><a href="../19437/environmental-impact-global-warming-1.html">Environmental impact - global warming</a></li><li><a href="../19453/electrochemistry-rechargeable-batteries-fuel-cells-1.html">Electrochemistry, rechargeable batteries & fuel cells</a></li><li><a href="../19458/nuclear-fusion-fission-hl--1.html">Nuclear fusion & fission (HL) </a></li><li><a href="../19470/photovoltaic-cells-and-dsscs-1.html">Photovoltaic cells and DSSCs</a></li></ul></li><li><a href="../19078/option-d-1.html" class="father">Option D</a><ul class="level-2"><li><a href="../19079/pharmaceutical-products-drug-action--1.html">Pharmaceutical products & drug action </a></li><li><a href="../19158/aspirin-penicillin--1.html">Aspirin & penicillin </a></li><li><a href="../19174/opiates-1.html">Opiates</a></li><li><a href="../19184/ph-regulation-of-the-stomach-1.html">pH regulation of the stomach</a></li><li><a href="../19194/antiviral-medications--1.html">Antiviral medications </a></li><li><a href="../19211/environmental-impact-of-some-medications-1.html">Environmental impact of some medications</a></li><li><a href="../19254/taxol-a-chiral-auxiliary-case-study-1.html">Taxol - a chiral auxiliary case study</a></li><li><a href="../19258/nuclear-medicine--1.html">Nuclear medicine </a></li><li><a href="../19273/drug-detection-analysis--1.html">Drug detection & analysis </a></li></ul></li></ul></li><li><a href="../16592/practical-scheme-of-work-ia--1.html">Practical scheme of work & IA </a><ul class="level-1"><li><a href="../16682/internal-assessment-1.html" class="father">Internal Assessment</a><ul class="level-2"><li><a href="../18892/scaffolding-the-investigation-1.html">'Scaffolding' the investigation</a></li><li><a href="../18896/timing-organisation--1.html">Timing & organisation </a></li><li><a href="../18905/choosing-the-research-question-1.html">Choosing the research question</a></li><li><a href="../18946/personal-engagement-1.html">Personal engagement</a></li><li><a href="../18950/exploration-1.html">Exploration</a></li><li><a href="../18957/analysis-1.html">Analysis</a></li><li><a href="../18965/evaluation-1.html">Evaluation</a></li><li><a href="../18966/communication-1.html">Communication</a></li><li><a href="../19882/internal-standardization-of-the-ia-1.html">Internal standardization of the IA</a></li><li><a href="../19901/submitting-the-samples-for-moderation-2.html">Submitting the samples for moderation</a></li><li><a href="../33754/ten-suggestions-for-ias-using-secondary-data-1.html">Ten suggestions for IAs using secondary data</a></li><li><a href="../37544/gaining-full-marks-for-a-databased-ia--1.html">Gaining full marks for a databased IA </a></li><li><a href="../19506/ia-example-marking-exercise--1.html">IA example & marking exercise </a></li><li><a href="../23929/genuine-examples-of-moderated-ia-reports-1.html">Genuine examples of moderated IA reports</a></li><li><a href="../25234/examples-of-teacher-marked-ia-reports--1.html">Examples of teacher-marked IA reports </a></li><li><a href="../37542/history-of-internal-assessment-1.html">History of Internal Assessment</a></li></ul></li><li><a href="../16598/mandatory-laboratory-components-1.html" class="father">Mandatory laboratory components</a><ul class="level-2"><li><a href="../16613/formula-of-magnesium-oxide--1.html">Formula of magnesium oxide </a></li><li><a href="../16612/determining-the-mr-of-an-unknown-gas-1.html">Determining the <i>M</i><sub>r</sub> of an unknown gas</a></li><li><a href="../16615/acid-base-titrations--1.html">Acid-base titrations </a></li><li><a href="../16616/a-green-acid-base-practical-1.html">A green acid-base practical</a></li><li><a href="../16617/analysis-of-aspirin-tablets-1.html">Analysis of aspirin tablets</a></li><li><a href="../16618/caco3-in-egg-shells-1.html">CaCO<sub>3</sub> in egg shells</a></li><li><a href="../16644/enthalpy-changes-1.html">Enthalpy changes</a></li><li><a href="../16631/reaction-rates-1.html">Reaction rates</a></li><li><a href="../16635/rate-dependent-factors-1.html">Rate-dependent factors</a></li><li><a href="../16636/determining-ea-for-a-reaction-1.html">Determining <i>E</i><sub>a</sub> for a reaction</a></li><li><a href="../16637/iodination-of-propanone-1.html">Iodination of propanone</a></li><li><a href="../18144/titrations-with-a-ph-meter-1.html">Titrations with a pH meter</a></li><li><a href="../18355/redox-titration-with-kmno4--1.html">Redox titration with KMnO<sub>4</sub> </a></li><li><a href="../16640/voltaic-cells-1.html">Voltaic cells</a></li><li><a href="../16659/3-d-molecular-modelling--1.html">3-D molecular modelling </a></li></ul></li><li><a href="../17168/other-good-practicals-1.html" class="father">Other good practicals</a><ul class="level-2"><li><a href="../17196/common-chemical-reactions-1.html">Common chemical reactions</a></li><li><a href="../227/elements-oxides-of-the-third-period--1.html">Elements & oxides of the third period </a></li><li><a href="../17239/the-halogens-1.html">The halogens</a></li><li><a href="../17162/boiling-points-of-mixtures-1.html">Boiling points of mixtures</a></li><li><a href="../17170/polarity-of-molecules-1.html">Polarity of molecules</a></li><li><a href="../18083/le-chateliers-principle--1.html">Le Chatelier's principle </a></li><li><a href="../19582/determining-kc-for-an-esterification-reaction-1.html">Determining <i>K</i><sub>c</sub> for an esterification reaction</a></li><li><a href="../18164/redox-reactions-of-vanadium--1.html">Redox reactions of vanadium </a></li><li><a href="../18351/chlorine-in-swimming-pools--1.html">Chlorine in swimming pools </a></li><li><a href="../18178/analysis-of-cuii-ions-in-solution--1.html">Analysis of Cu(II) ions in solution </a></li><li><a href="../18179/percentage-of-copper-in-brass-1.html">Percentage of copper in brass</a></li><li><a href="../18356/electrolytic-cells--1.html">Electrolytic cells </a></li><li><a href="../18598/reactions-of-organic-compounds-1.html">Reactions of organic compounds</a></li><li><a href="../18752/hydrolysis-of-halogenoalkanes-1.html">Hydrolysis of halogenoalkanes</a></li><li><a href="../18784/preparation-of-13-dinitrobenzene--1.html">Preparation of 1,3-dinitrobenzene </a></li><li><a href="../19022/determination-of-an-organic-structure-1.html">Determination of an organic structure</a></li><li><a href="../19887/preparation-of-nylon-66-1.html">Preparation of nylon 6,6</a></li><li><a href="../19343/determination-of-vitamin-c-content--1.html">Determination of vitamin C content </a></li><li><a href="../19342/hydrolysis-of-starch--1.html">Hydrolysis of starch </a></li><li><a href="../19159/preparation-purification-of-aspirin--1.html">Preparation & purification of aspirin </a></li></ul></li><li><a href="../18851/ict-in-practical-work-1.html" class="father">ICT in practical work</a><ul class="level-2"><li><a href="../18852/data-logging--1.html">Data logging </a></li><li><a href="../18853/software-for-graph-plotting--1.html">Software for graph plotting </a></li><li><a href="../18854/spreadsheets-for-data-processing--1.html">Spreadsheets for data processing </a></li><li><a href="../18855/databases-1.html">Databases</a></li><li><a href="../18856/computer-modelling-simulations--1.html">Computer modelling & simulations </a></li></ul></li><li><a href="../16594/group-4-project-1.html" class="father">Group 4 Project</a><ul class="level-2"><li><a href="../16595/practicalities-1.html">Practicalities</a></li><li><a href="../16596/reflective-statement--2.html">Reflective statement </a></li></ul></li><li><a href="../16666/a-new-laboratory-1.html">A new laboratory?</a></li></ul></li><li class="selected"><a href="../16284/exams-1.html">Exams</a><ul class="level-1"><li><a href="../16286/essential-facts-1.html" class="father">Essential Facts</a><ul class="level-2"><li><a 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concepts</a></li><li><a href="../3507/why-tok-chemistry-1.html">Why TOK & Chemistry?</a></li><li><a href="../3505/a-modern-paradigm-1.html">A modern paradigm</a></li></ul></li></ul></li><li><a href="../17783/fast-track-to-tests-questions-1.html">Fast track to tests & questions</a><ul class="level-1"><li><a href="../17784/sl-multiple-choice-tests-on-individual-topics-1.html">SL Multiple choice tests on individual topics</a></li><li><a href="../24351/sl-multiple-choice-quizzes-on-sub-topics-1.html">SL Multiple choice 'quizzes' on sub-topics</a></li><li><a href="../31519/sl-practice-paper-1-exams-1.html">SL Practice Paper 1 exams</a></li><li><a href="../17786/sl-questions-on-each-sub-topic-1.html">SL Questions on each sub-topic</a></li><li><a href="../20431/sl-paper-3-section-a-questions-1.html">SL Paper 3 Section A questions</a></li><li><a href="../17792/sl-questions-on-the-options-1.html">SL Questions on the options</a></li><li><a href="../24897/sl-quizzes-on-option-sub-topics--1.html">SL Quizzes on option sub-topics </a></li><li><a href="../17785/hl-multiple-choice-tests-on-individual-topics-1.html">HL Multiple choice tests on individual topics</a></li><li><a href="../24352/hl-multiple-choice-quizzes-on-sub-topics-1.html">HL Multiple choice 'quizzes' on sub-topics</a></li><li><a href="../31520/hl-practice-paper-1-exams--1.html">HL Practice Paper 1 exams </a></li><li><a href="../17787/hl-questions-on-each-sub-topic--1.html">HL Questions on each sub-topic </a></li><li><a href="../20428/hl-paper-3-section-a-questions-1.html">HL Paper 3 Section A questions</a></li><li><a href="../17793/hl-questions-on-the-options-1.html">HL Questions on the options</a></li><li><a href="../24973/hl-quizzes-on-option-sub-topics-1.html">HL Quizzes on option sub-topics</a></li><li><a href="../41573/printable-versions-of-written-tasks--1.html">Printable versions of written tasks </a></li></ul></li><li><a 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</div><section id="main-content"><div class="blueBg"><h1>The theory</h1><p>For some reason students seem to find it very difficult to predict the shape and bond angles of a simple molecule or ion using Valence Shell Electron Pair Repulsion Theory (<span data-scayt-word="VSEPR" data-wsc-lang="en_US">VSEPR</span>). At first sight this seems strange because the theory really is very simple. Essentially there are only three basic ideas:</p><hr class="hidden"><ul class="branded"><li>Electrons arrange themselves in pairs around the central atom or ion so that there is maximum repulsion between the pairs. Strictly speaking it is electron domains rather than electron pairs as this would take account of an unpaired electron in a free radical but this is not necessary at this level. In fact in previous syllabi the term 'negative charge <span data-scayt-word="centres" data-wsc-lang="en_US">centres</span>' was used but in sub-topic 4.3 : Covalent structures it clearly states that 'electron domain' should be used in place of 'negative charge centre'.<br> </li><li>Non-bonding pairs of electrons exert a greater repulsion than bonded pairs.<br> </li><li>A multiple bond (i.e. a double or triple bond) must count as one pair of electrons (one electron domain) as all the pairs of electrons must be aligned in the same direction.</li></ul><hr class="hidden"><p>Applying the first idea leads to the fact that there are only five basic shapes to be considered. In fact there are only three at Standard Level as sub-topic 4.3 limits the shape to two, three and four electron domains on the central atom. For Higher Level students sub-topic 14.1 extends this to five and six pairs. These basic shapes and bond angles are:</p><hr class="hidden"><table align="center" border="1" cellpadding="20" cellspacing="2"><tbody><tr><td style="background-color: rgb(153, 204, 204);"><strong>Number of pairs of electrons</strong></td><td style="background-color: rgb(153, 204, 204);"><strong>Shape</strong></td><td style="background-color: rgb(153, 204, 204);"><strong>Bond angle(s)</strong></td></tr><tr><td style="text-align: center">2</td><td>Linear</td><td>180<sup>o</sup></td></tr><tr><td style="text-align: center">3</td><td>Trigonal planar</td><td>120<sup>o</sup></td></tr><tr><td style="text-align: center">4</td><td>Tetrahedral</td><td>109.5<sup>o</sup> (actually 109<sup>o</sup> 28')</td></tr><tr><td style="text-align: center">5</td><td>Trigonal bipyramidal</td><td>90<sup>o</sup>, 120<sup>o</sup>, 180<sup>o</sup></td></tr><tr><td style="text-align: center">6</td><td>Octahedral</td><td>90<sup>o</sup>, 180<sup>o</sup></td></tr></tbody></table><hr class="hidden"><p>If one or more pairs are non-bonded then the basic shape adapts to take account of this. An example is ammonia.</p><hr class="hidden"><p><img alt="" class="left noborder" src="../../../ib/chemistry/images/Exams/ammonia shape.png" style="width: 217px; height: 189px; float: left;" title="Trigonal pyramidal shape of a molecule of ammonia">There are four pairs of electrons around the central nitrogen atom. Three of these are bonding and one is non-bonding. Four pairs gives a tetrahedral shape but as one of the pairs is a non-bonding pair the actual shape will be trigonal pyramid (with the non-bonded pair pointing to the apex of what would have been the tetrahedron). As the non-bonding pair exerts a greater repulsion than bonding pairs this forces the bonded pairs slightly closer together and the H-N-H bond angle is reduced from the 109.5<sup>o </sup> of a regular tetrahedron to approximately 107<sup>o</sup>. Other examples follow a similar logic and basically that is all there is to it - at least as far as the IB goes. In fact it is not nearly so simple when you try to extend it to other molecules and ions. You can read more about this in 'Pause for Thought' on the page covering shapes of molecules or ions in <a href="../17122/covalent-structures-2-1.html" title="Core & AHL » Teaching each topic & sub-topic » Topics 4 & 14 » Covalent structures (2)">Covalent structures (2)</a>. </p><hr class="hidden"></div><hr class="hidden"><div class="greenBg"><h1>The problem for students</h1><p>So why do students seem to find it so difficult? I think there are two fundamental reasons. The first is that they are not clear about how to determine the number of electron pairs around the <strong>central</strong> atom. To do this they need to understand about Lewis structures but then ignore all the electron pairs around all the outer atoms. At Standard Level they can use the octet rule to give them the number and almost all examples will be four pairs due to the eight electrons around the central atom. However, because some of the pairs might be a double or triple bond this will reduce the number of pairs according to <span data-scayt-word="VSEPR" data-wsc-lang="en_US">VSEPR</span> theory. So that, for example, a carbon dioxide molecule which has the eight electrons around the central carbon atom arranged in two double bonds will have two pairs according to <span data-scayt-word="VSEPR" data-wsc-lang="en_US">VSEPR</span> and is linear. </p><p>The second reason is that they are unable to visualize the shapes in three dimensions. There is an interesting article about this written by Ashley Jennings, a high school student from Pennsylvania, in <a href="/media/ib/chemistry/files/VSEPR%20article.pdf">Journal of Chemical Education, Vol. 87 No. 5 May 2010 pages 462-463</a>. In this article Ashley talks about how difficult it was for students to equate two-dimensional drawings with the three-dimensional models the teacher had <strong>shown</strong> them. Ashley's solution to this was to develop her own video showing the models in three dimensions.</p><hr class="hidden"><p style="text-align: center"><iframe allowfullscreen="" frameborder="0" height="280" src="//www.youtube.com/embed/i3FCHVlSZc4?rel=0" width="360"></iframe></p><p style="text-align: center;"><a href="http://youtu.be/i3FCHVlSZc4">VSEPR theory <img class="ico" src="../../../img/icons/video-1.png"><span class="anchor" id="video-1" name="video-1"> </span></a></p><hr class="hidden"><p>This is a well-constructed video and students may find it helpful. I think though that the real problem is that often students are only <strong>shown</strong> the shapes and even in this video the shapes are <strong>shown</strong> on a two-dimensional screen. I have found that students gain a much better understanding and concept of the shapes when they <strong>make their own models</strong>. An investment in molecular modelling kits, <a href="http://www.molymod.com/">e.g. molymod </a>(one for each pair of students) is essential. If students always have these kits at hand then whenever a shape is discussed they can make it for themselves and feel it in three-dimensions.</p><p>Finally, if they are still having problems, it might be worth them just learning the shapes of some specific molecules and ions as the examples the IB can use are quite limited in number! For those that do finally 'get it' you might like to extend their understanding with ideas from the page on extending the use of the data booklet to <a href="../17378/shapes-and-hybridization--1.html" title="Basic information » Resources » Novel uses for the IB data booklet » Shapes and hybridization ">Shapes and hybridization</a>.</p><hr class="hidden"></div></section></article><section id="media-extras"><div class="page-actions no-print navbar inline hidden-desktop"><div class="navbar-inner"><ul class="nav"><li><a class="presentation" href="#" onclick="return false;"><i class="fa fa-desktop"></i></a></li><li><a class="print-section-blog" href="#" onclick="return false;"><i class="fa fa-print"></i></a></li><li><a class="page-bookmarker" data-ticket="IB Docs (2) Team" data-pid="16311" href="#" onclick="return false;"><i class="fa fa-star"></i></a></li><li><a class="personal-notes" href="#" onclick="return false;"><i class="fa fa-file-text"></i></a></li><li><a class="" data-toggle="modal" href="#modal-feedback" onclick="return false;"><i class="fa fa-envelope-o"></i></a></li><li class="dropdown"><a class="dropdown-toggle" data-toggle="dropdown" data-target="#" href="#"><i class="fa fa-share-alt"></i></a><ul class="dropdown-menu" role="menu"><li><a class="" target="_blank" title="Share on Twitter" href="https://twitter.com/share?text=%22VSEPR%22+-+via+%40InThinker+%23chemistry%0A&url=https%3A%2F%2Fwww.thinkib.net%2Fchemistry%2Fpage%2F16311%2Fvsepr"><i class="fa fa-twitter-square"></i><span>Twitter</span></a></li><li><a class="" target="_blank" title="Share on Facebook" href="https://www.facebook.com/sharer.php?u=https%3A%2F%2Fwww.thinkib.net%2Fchemistry%2Fpage%2F16311%2Fvsepr&t=VSEPR"><i class="fa fa-facebook-square"></i><span>Facebook</span></a></li><li><a class="" href="http://www.linkedin.com/shareArticle?mini=true&url=https%3A%2F%2Fwww.thinkib.net%2Fchemistry%2Fpage%2F16311%2Fvsepr"><i class="fa fa-linkedin-square"></i><span>LinkedIn</span></a></li><li><a class="" href="https://plus.google.com/share?url=https%3A%2F%2Fwww.thinkib.net%2Fchemistry%2Fpage%2F16311%2Fvsepr"><i class="fa fa-google-plus-square"></i><span>Google +</span></a></li><li><a class="" href="mailto:?subject=VSEPR&body=For some reason students seem to find it very difficult to predict the shape and bond angles of a simple molecule or ion using Valence Shell Electron Pair Repulsion Theory (VSEPR). 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