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Double bond rule
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<p>In <a href="/facts/Chemistry/SrNqkGVm">chemistry</a>, the double bond rule states that <a href="/facts/Chemical_element/0jducgWD">elements</a> with a <a href="/facts/Principal_quantum_number/8SzSWV6w">principal quantum number</a> (<i>n</i>) greater than 2 for their <a href="/facts/Valence_electron/mXPA15VP">valence electrons</a> (<a href="/facts/Period_3_element/1BE0Nc0K">period 3 elements</a> and higher) tend not to form multiple bonds (e.g. <a href="/facts/Double_bond/qDemMn0c">double bonds</a> and <a href="/facts/Triple_bond/poDWRayX">triple bonds</a>). Double bonds for these heavier elements, when they exist, are often weak due to poor <a href="/facts/Orbital_overlap/IECydUNY">orbital overlap</a> between the <i>n</i>>2 orbitals of the two atoms. Although such compounds are not intrinsically unstable, they instead tend to dimerize or even <a href="/facts/Polymerize/1hmBz7Ur">polymerize</a>. Moreover, the multiple bonds of the elements with <i>n</i>=2 are much stronger than usual, because lone pair repulsion weakens their sigma bonding but not their pi bonding. An example is the rapid polymerization that occurs upon condensation of <a href="/facts/Disulfur/otzhNhox">disulfur</a>, the heavy analogue of O2. Numerous exceptions to the rule exist. Several exceptions of this rule has been already made. </p> Double bonds for carbon and nearest neighbours<table><tbody><tr><th></th><th>B<a href="/facts/Boron/Pmwgd7Mf">boron</a>(<i>n</i>=2)</th><th>C<a href="/facts/Carbon/ukrgQexo">carbon</a>(<i>n</i>=2)</th><th>N<a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>(<i>n</i>=2)</th><th>O<a href="/facts/Oxygen/acyn6o8r">oxygen</a>(<i>n</i>=2)</th><th>Si<a href="/facts/Silicon/cgWx0hdr">silicon</a>(<i>n</i>=3)</th><th>P<a href="/facts/Phosphorus/27xCeIqs">phosphorus</a>(<i>n</i>=3)</th><th>S<a href="/facts/Sulfur/IK0amTfM">sulfur</a>(<i>n</i>=3)</th></tr><tr><th>B</th><td><a href="/facts/Diborene/r6NQHXYt">diborenes</a></td><td><a href="/facts/Alkylideneborane/r6NQHXYt">alkylideneboranes</a></td><td>aminoboranylidenes, rare</td><td><a href="/facts/Oxoborane/ACmMrEsV">oxoboranes</a>, rare, rapid oligomerization</td><td>borasilenes (rare)</td><td>boranylidenephosphanes, rare, stable compounds are known</td><td>thioxoboranes, rare</td></tr><tr><th>C</th><td></td><td><a href="/facts/Alkene/RtYcS6kU">alkenes</a></td><td><a href="/facts/Imine/yK72z07j">imines</a></td><td><a href="/facts/Carbonyl/KJ71bo3X">carbonyls</a></td><td><a href="/facts/Silenes/mSr8aF2N">silenes</a></td><td><a href="/facts/Phosphaalkene/IzpJHbUH">phosphaalkenes</a></td><td><a href="/facts/Thioketone/wMf09spv">thioketones</a></td></tr><tr><th>N</th><td colspan="2"></td><td><a href="/facts/Azo_compound/dnl1WKnK">azo compounds</a></td><td><a href="/facts/Nitroso/Irref1C6">nitroso compounds</a></td><td>silanimines, rare, easy oligomerization, observed only at low temp</td><td><a href="/facts/Phosphazene/IR0jFetb">phosphazene</a> (P=N)</td><td><a href="/facts/Sulfilimine/fHBlbU8h">sulfilimines</a></td></tr><tr><th>O</th><td colspan="3"></td><td><a href="/facts/Singlet_oxygen/db0PsrYh">Singlet oxygen</a></td><td><a href="/facts/Silanones/SI1h9e6q">silanones</a>, Si=O bonds extremely reactive, oligomerization to <a href="/facts/Siloxane/6fbEIal7">siloxanes</a></td><td>numerous, e.g. <a href="/facts/Phosphine_oxide/Tq9TMtn4">phosphine oxides</a>, <a href="/facts/Phosphonate/8mg6nNRP">phosphonates</a>, <a href="/facts/Phosphinate/Je5Gckyh">phosphinates</a>,<a href="/facts/Phosphate/FDGVCxUG">phosphates</a></td><td>numerous, e.g. <a href="/facts/Sulfuric_acid/Wom6fymJ">sulfuric acid</a>, <a href="/facts/Sulfates/yuWVxsUf">sulfates</a>, <a href="/facts/Sulfoxide/Stk3Vnj9">sulfoxides</a> (R-S(=O)-R′, compounds with a <a href="/facts/Sulfinyl/Stk3Vnj9">sulfinyl</a> group), and <a href="/facts/Sulfone/LJLP1RcT">sulfones</a> (R-S(=O)2-R′, the <a href="/facts/Sulfonyl_group/UbpYXVvN">sulfonyl group</a>)</td></tr><tr><th>Si</th><td colspan="4"></td><td><a href="/facts/Disilene/yknxER4V">disilenes</a></td><td>silylidenephosphanes a.k.a. <a href="/facts/Phosphasilenes/cTCeYUXx">phosphasilenes</a>, rare</td><td>silanethiones, rare, easy oligomerization</td></tr><tr><th>P</th><td colspan="5"></td><td><a href="/facts/Diphosphenes/cT2bPDXu">diphosphenes</a></td><td>common compounds such as <a href="/facts/Thiophosphate/djY5nEmv">thiophosphates</a> and phosphine sulfides, for example, <a href="/facts/Triphenylphosphine_sulfide/GGY92EGK">triphenylphosphine sulfide</a> and certain <a href="/facts/Dithiadiphosphetane/4YLlrBT3">dithiadiphosphetanes</a></td></tr><tr><th>S</th><td colspan="6"></td><td><a href="/facts/Disulfur/otzhNhox">disulfur</a>, <a href="/facts/Thiosulfoxide/a6EJWfm5">thiosulfoxides</a></td></tr></tbody></table>