Atomic and molecular models
A pompom model of a molecule of methane
These pompom models complement traditional 'ball and stick' versions and extend their scope by
introducing an element of fuzziness and squashability into the atoms. It is also easy to change bond
angles as required, because the bonds are mounted flexibly within the atom. Models can be made in
the traditional CMK colour scheme, and in size ratios to reflect the differences in atomic radii. Further
sophistication could be introduced if desired, for example by preparing different models for different
ionic radii, or by adding elastic to represent bond vibrations and weaker connectors to represent
hydrogen bonds. The bonds consist of pipe cleaners, cut to size and with ends folded down for
protection from the sharp points.
You will need:
Templates. These can be made from card or plastic. You need two per pompom, and if you are
careful, they can be reused. The CLEAPSS models used outer diameters of 2.5 cm for the
hydrogen models and 5.0 cm for carbon, oxygen, nitrogen and sulfur.
o Wool
o Pipe cleaners, cut to size. Two bonds can be represented by one double length of pipe cleaner.
o Metal or plastic washers: one per two bonds in a model.
o Scissors
o A needle (optional; can be helpful when inserting the final rounds of wool).
Instructions
o
1 Place two templates together. Cut a length of wool. You can use double or quadruple wool to speed
the process, but avoid making the wool so long that it tangles.
2 Secure the end of the wool by sandwiching it between the templates, then start winding it round
and round the templates. When the wool is used up, take a fresh length. Wind over the end of the first
length and the new length, then continue winding till that strand is also used up. Continue with fresh
strands until you can no longer fit more around the template. The hole will be filled and the wound
wool will be filled out into a fat shape.
3 Carefully insert the tip of the scissors through the wound wool and between the templates. Cut
through the wool all round, creating separate strands.
4 At this point, you can insert the bonds. Cut the appropriate number of pieces of pipe cleaner. Fold
CLEAPSS Draft Atomic Models January 2014
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down the sharp ends for protection. To secure the bonds, you can insert one end of the piece of pipe
cleaner (or its centre if you are making two bonds) into a washer, and place the washer in the centre
of the wound pompom.
5 Insert another piece of wool between the templates. Wind it tightly round the centres of the cut
fibres, pulling it tight to secure them. Tie this wool securely.
6 Carefully slip each template off the wool, and trim the strands to make a good shape.
Arrange the bonds at appropriate angles to represent the bonds.
7 Bond two models together by twisting the pipe cleaners from two pompoms together. Double bonds
are made when two bonds from one model are involved.
Starting the pompom
Winding the yarn and
securing the ends
Inserting the bonds
Cutting and tying the
pompom
Template for pompom atoms or ions
CLEAPSS simply used two sizes of template, but see the table below for details of actual atomic and
ionic radii if you wish to make more accurate models.
Large template
Small template
Atomic and ionic radii and CPK colours
The CPK scheme is a standard colour scheme for atomic models.
Element
CPK colour
White
Atomic radius
(nm)
0.12
Covalent radius
(nm)
0.037
Hydrogen
Carbon
Black
0.17
0.077
Nitrogen
Dark blue
0.16
0.075
Oxygen
Red
0.15
0.073
Sulfur
Yellow
0.18
0.102
Sodium
Violet
0.23
Chlorine
Green
0.18
Other metals
Grey
Various
CLEAPSS Draft Atomic Models January 2014
Ionic radius, most
common ion (nm)
0.102
0.099
0.180
2