Shapes of molecules IWORKING OUT THE SHAPES OFPARTICLESCONTAINING ONLY SINGLE BONDSThe number of electron pairs in molecules containing only singlebonds is found by following these steps:1. Decide which atom is at the centre of the molecule (usually theatom of which there is only one).2. Look up which periodic group it is in: this gives the number ofouter electrons it has.3. Add an electron if the particle is negatively charged; subtractan electron if the particle is positively charged.4. Add one electron for each atom joined to the central atom.5. Divide by two to get the number of pairs.6. This gives the shape of the electron pairs. Now see if there arethe same number of bonded atoms; if not, the extra pairs arelone pairs.7. Now name the shape of the molecule which is defined only bythe arrangement of nuclei
11 Shapes of molecules I The number of electron pairs in molecules containing only single bonds is found by following these steps: 1. Decide which atom is at the centre of the molecule (usually the atom of which there is only one). 2. Look up which periodic group it is in: this gives the number of outer electrons it has. 3. Add an electron if the particle is negatively charged; subtract an electron if the particle is positively charged. 4. Add one electron for each atom joined to the central atom. 5. Divide by two to get the number of pairs. 6. This gives the shape of the electron pairs. Now see if there are the same number of bonded atoms; if not, the extra pairs are lone pairs. 7. Now name the shape of the molecule which is defined only by the arrangement of nuclei. WORKING OUT THE SHAPES OF PARTICLES CONTAINING ONLY SINGLE BONDS
Shapes of molecules IEXAMPLESOFWORKINGOUTTHESHAPESOF MOLECULESPCI+SCIAPClsNHSH2OStepsNOSHtetrahedralsquaredistortedtrigonaltrigonalbentplanaryramidalbipyramidaltetrahedron12
12 Shapes of molecules I
Shapes of molecules IWordsWordsrepel: repulsionlone pairs; shared pairs;bondingpairslineartrigonalplanar;squareplanartrigonal bipyramidal; trigonal pyramidal; square pyramidaltetrahedral:tetrahedron,tetrahedraoctahedral:octahedron,octahedradistorted:distortbent13
13 Shapes of molecules I Words repel: repulsion lone pairs; shared pairs; bonding pairs linear trigonal planar;square planar trigonal bipyramidal; trigonal pyramidal; square pyramidal tetrahedral: tetrahedron, tetrahedra octahedral: octahedron, octahedra distorted: distort bent Words
Shapes ofmoleculesIlEXPLAINING DIFFERENTBONDANGLESThe exact angles between bonds can be found using X-raydiffraction.These angles are evidence for the idea that lone pairsrepel more than bonding pairs. For example, the angles inammonia and water are less than those in methane, as shownbelow.HgreaterevengreaterrepulsionrepulsionC0HHHHHHH109°107°105°14
14 Shapes of molecules II The exact angles between bonds can be found using X-ray diffraction. These angles are evidence for the idea that lone pairs repel more than bonding pairs. For example, the angles in ammonia and water are less than those in methane, as shown below. EXPLAINING DIFFERENT BOND ANGLES
Shapes of molecules IIWORKINGOUTTHESHAPESOFOXO-COMPOUNDSMany molecules and ions contain oxygen. This oxygen is:sometimes double bonded and sometimes single bonded. inwhich case the oxygen atom carries a minus charge.The shape of these particles can be worked out by following thesesteps:1. For each minus charge on the particles there is one single bondedoxygen atom with a minus charge.2. All the other oxygens are double bonded to the central atom. Thisdouble bond counts as one electron charge cloud when working outshapes.3. So draw the two types of oxygen around the central atom andcheck the number of electrons it is using in bonds to the oxygens.4. If there are any unused lone pairs around the central atom putthem in, then choose the shapes for the number of electron chargeclouds around the central atom15
15 Shapes of molecules II The shape of these particles can be worked out by following these steps: 1. For each minus charge on the particles there is one single bonded oxygen atom with a minus charge. 2. All the other oxygens are double bonded to the central atom. This double bond counts as one electron charge cloud when working out shapes. 3. So draw the two types of oxygen around the central atom and check the number of electrons it is using in bonds to the oxygens. 4. If there are any unused lone pairs around the central atom put them in, then choose the shapes for the number of electron charge clouds around the central atom. WORKING OUT THE SHAPES OF OXO-COMPOUNDS Many molecules and ions contain oxygen. This oxygen is: sometimes double bonded and sometimes single bonded, in which case the oxygen atom carries a minus charge