Molecular Shapes

Lewis Diagram Allow You to Determine the Three-dimensional

Shape of a Molecule

Molecular Shape is Important!

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

H O H

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

Water

H O H O H

H

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

Bent water is, of course, a liquid; shape matters!

Molecular Shape is Important

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 1-2-3

Actually, not quite... It’s as easy as 2-3-4, instead! We only need to count GROUPS of

electrons around certain atoms to determine shape in the vicinity of that atom

Electron Groups

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

Each of the above counts as a single electron group

Electron Groups

Electron Groups

Electron Groups

Electron Groups

Electron Groups

Four groups of ElectronsFour groups of Electrons

Electron Groups

N:N:HH

HH

HH

Electron Groups

N:N:HH

HH

HHone groupone group

Electron Groups

N:N:HH

HH

HHone groupone group

one groupone group

Electron Groups

N:N:HH

HH

HHone groupone group

one groupone group

one groupone group

Electron Groups

N:N:HH

HH

HH

one groupone group

one groupone group

one groupone group

one groupone group

Four groups of electrons.Four groups of electrons.

Lone pairs count!

Electron Groups

O:O:HH

HH

.. ..

Electron Groups

O:O:HH

HH

.. ..

one groupone group

Electron Groups

O:O:HH

HH

.. ..

one groupone group

one groupone group

Electron Groups

O:O:HH

HH

.. ..

one groupone group

one groupone group one groupone group

one groupone group

Again, lone pairs count.Again, lone pairs count.

Electron Groups

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

Electron Groups

NH C :

one group

one group

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

Determining Shapes

Use the Lewis diagram to establish shapeUse the Lewis diagram to establish shape The shape is described at each central (pivot, The shape is described at each central (pivot,

“hinge”) atom in the molecule“hinge”) atom in the molecule Some molecules may have different shapes at Some molecules may have different shapes at

different central atomsdifferent central atoms Don’t worry about shape at outside atomsDon’t worry about shape at outside atoms Our molecules will usually have only one Our molecules will usually have only one

central atom central atom

Determining Shapes

H O

H

:. .

central atom (shape important)

outside atom (no shape)

Determining Shapes

CN

. .

shape important (central, pivot atoms)

shape not important(outside atoms)

H

Determining Shapes

The The number of electron groupsnumber of electron groups around a around a central atom determines its shapecentral atom determines its shape

The shape will correspond to a 3-D The shape will correspond to a 3-D arrangement which allows maximum arrangement which allows maximum separation of each electron groupseparation of each electron group

Remember, an electron group is: Remember, an electron group is: – a lone paira lone pair– a bond group (single, double, triple bond)a bond group (single, double, triple bond)

Two electron groups

For octet-rule atoms (ours are!), this will be For octet-rule atoms (ours are!), this will be eithereither– one single bond + one triple bondone single bond + one triple bond

HCN:

2 electron groups

Two electron groups

For octet-rule atoms (ours are!), this will be For octet-rule atoms (ours are!), this will be eithereither– one single bond + one triple bondone single bond + one triple bond– two double bondstwo double bonds

HCN:

2 electron groups

:O=C=O:. .. .

2 electron groups

Two electron groups

Two electron groups generate a Two electron groups generate a LINEARLINEAR (straight-line) geometry (180°) separation(straight-line) geometry (180°) separation

Atoms connected to the linear electron Atoms connected to the linear electron groups lie in their same directiongroups lie in their same direction

HCN:

180° separation

:O=C=O:. .. .

180° separation = linear

Three Electron Groups For octet-rule atoms this combination will For octet-rule atoms this combination will

be:be:– two single bonds + a double bondtwo single bonds + a double bond– one single bond + one double bond + one lone one single bond + one double bond + one lone

pair pair one group

one group

one group

(lone pairs at O, Cl omitted,for clarity)

Three Electron Groups Three electron groups generate a Three electron groups generate a

TRIGONALTRIGONAL (triangle-like) geometry (triangle-like) geometry– electron groups and their atoms are about 120° electron groups and their atoms are about 120°

apartapart

Four Electron Groups

Result from:Result from:– four single bondsfour single bonds– three single bonds + a lone pairthree single bonds + a lone pair– two single bonds and two lone pairstwo single bonds and two lone pairs– never multiple bondsnever multiple bonds

Four Electron Groups Result from:Result from:

– four single bondsfour single bonds

Four Electron Groups Result from:Result from:

– four single bondsfour single bonds– three single bonds + a lone pairthree single bonds + a lone pair

4 groups, including lone pair

Four Electron Groups Result from:Result from:

– four single bondsfour single bonds– three single bonds + a lone pairthree single bonds + a lone pair– two single bonds and two lone pairstwo single bonds and two lone pairs

4 groups, including lone pair

H

..

4 groups:count ‘em...

Four Electron Groups

4 Electron groups generate a 4 Electron groups generate a TETRAHEDRALTETRAHEDRAL geometry, named after geometry, named after tetrahedron, a figure from 3-D geometrytetrahedron, 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

Four groups of Electrons

N:

Four groups of Electrons

O

H:

: H