Molecular Shapes Lewis Diagram Allow You to Determine the Three-dimensional Shape of a Molecule

Molecular Shape is Important! u u Shape influences physical properties – –Affects features such as boiling point, physical state, hardness, etc – –Water, for example, would surely be a gas at room temperature (and well below) if its shape were linear – –Instead water’s bent shape makes it a liquid at ordinary temperatures

Water If water had a linear shape, it would be a gas!

Water If water had a linear shape, it would be a gas! Bent water is, of course, a liquid; shape matters!

Molecular Shape is Important u u Shape influences chemical properties – –Affects rate of reaction – –Affects site within a molecule where reaction might occur – –Affects type of catalyst or enzyme which might be operative with reaction

Determining a Molecule’s Shape: It’s as Easy as u u Actually, not quite... u u It’s as easy as 2-3-4, instead! u u We only need to count GROUPS of electrons around certain atoms to determine shape in the vicinity of that atom

Electron Groups u u An electron group is a set of electrons that are in the same area of the molecule – –The two electrons of a lone pair – –The two electrons of a single bond – –The four electrons of a double bond – –The six electrons of a triple bond u u Each of the above counts as a single electron group

Electron Groups

Four groups of Electrons

Electron Groups N: H H H

N: H H H one group

Electron Groups N: H H H one group

Electron Groups N: H H H one group

Electron Groups N: H H H one group Four groups of electrons. Lone pairs count!

Electron Groups O: H H..

O: H H.. one group

Electron Groups O: H H.. one group

Electron Groups O: H H.. one group Again, lone pairs count.

Electron Groups There are three electron groups around carbon. Double bond counts as one group.

Electron Groups one group Around carbon, there are only two electron groups; the triple bond is one group.

Determining Shapes u Use the Lewis diagram to establish shape u The shape is described at each central (pivot, “hinge”) atom in the molecule u Some molecules may have different shapes at different central atoms u Don’t worry about shape at outside atoms u Our molecules will usually have only one central atom

Determining Shapes H O H :.. central atom (shape important) outside atom (no shape)

Determining Shapes    C  N   .. shape important (central, pivot atoms) shape not important (outside atoms) H

Determining Shapes u The number of electron groups around a central atom determines its shape u The shape will correspond to a 3-D arrangement which allows maximum separation of each electron group u Remember, an electron group is: –a lone pair –a bond group (single, double, triple bond)

Two electron groups u For octet-rule atoms (ours are!), this will be either –one single bond + one triple bond H  C  N: 2 electron groups

Two electron groups u For octet-rule atoms (ours are!), this will be either –one single bond + one triple bond –two double bonds H  C  N: 2 electron groups :O=C=O: electron groups

Two electron groups u Two electron groups generate a LINEAR (straight-line) geometry (180°) separation u Atoms connected to the linear electron groups lie in their same direction H  C  N: 180° separation :O=C=O: ° separation = linear

Three Electron Groups u For octet-rule atoms this combination will be: –two single bonds + a double bond –one single bond + one double bond + one lone pair one group (lone pairs at O, Cl omitted, for clarity)

Three Electron Groups u Three electron groups generate a TRIGONAL (triangle-like) geometry –electron groups and their atoms are about 120° apart

Four Electron Groups u Result from: –four single bonds –three single bonds + a lone pair –two single bonds and two lone pairs –never multiple bonds

Four Electron Groups u Result from: –four single bonds

Four Electron Groups u Result from: –four single bonds –three single bonds + a lone pair      4 groups, including lone pair

Four Electron Groups u Result from: –four single bonds –three single bonds + a lone pair –two single bonds and two lone pairs      4 groups, including lone pair H   .. 4 groups: count ‘em...

Four Electron Groups u 4 Electron groups generate a TETRAHEDRAL geometry, named after tetrahedron, a figure from 3-D geometry X It is as if the tetrahedral atom were “buried” in the tetrahedron. An angle of about 109° exists between corners and the central atom 109°

Four groups of Electrons

N :

O H : : H