How Do We Predict Bond Angles and the Shapes of Molecules

Balloon models used to predict bond angles. (a)Two balloons assume a linear shape with a bond angle of 180° about the tie point. (b)Three balloons assume a trigonal planar shape with bond angles of 120° about the tie point, (c) Four balloons assume a tetrahedral shape with bond angles of 109.5° about the tie point. 

The shape of a methane molecule, CH4. (a) Lewis structure and (b) ball-and-stick model.The single bonds occupy four regions of electron density, causing the molecule to be tetrahedral. The hydrogens occupy the four corners of a regular tetrahedron, and all H — C—H bond angles are 109.5°. 

A general prediction emerges from this discussion of the shapes of CH4, NH3, and HgO molecules. If a Lewis structure shows four regions of electron density around an atom, then VSEPR predicts a tetrahedral distribution of electron density and bond angles of approximately 109.5°. 

Shapes of formaldehyde (CH2O) and ethylene (C2H4). In each molecule, the carbons are surrounded by three regions of electron density.Three regions of electron density about an atom are farthest apart when they lie in a plane and make angles of 120° with each other. We describe the geometry about each carbon atom as trigonal planar. 

Shapes of (a) carbon dioxide (CO2) and (b) acetylene (C2H2). In each case, the two regions of electron density are farthest apart if they form a straight line through the central atom and create an angle of 180°. Both carbon dioxide and acetylene are referred to as linear molecules. 

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VSEPRT seems to work for simple structures but surely there must be more to it than this Indeed there is. If we really want to understand why molecules adopt the shapes they do, we must look at the atoms that make up the molecules and how they combine. By the end of this chapter, you should be able to predict or at least understand the shapes of simple molecules. For example, why are the bond angles in ammonia 107°, while in hydrides of the other elements in the same group as nitrogen, PH3, AsH3, and SbH3> they are all aronnd 90° Simple VSEPRT would suggest tetrahedral arrangements for each. 

Let us count electrons now and ascertain that the atoms in the water molecule reached Nirvana. Thus, it is seen that O is surrounded by four electron pairs, two lone pairs, and two bond pairs, and hence it attains octet within the molecule. Each hydrogen atom has a duet, so all the atoms in the molecule achieved Nirvana, and the molecule is stable. Note that we draw the molecule with a bent angle. The water molecule is indeed bent, and this is very important for its ability to aggregate with many other molecules and make the liquid water we all know well. But at this stage, we still do not have the know-how to predict that this would be the shape of the molecule. We will Until then, it is also OK if you draw your water as linear H-O-H. 

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