An explanation sheet for making

Straw Molecular Model

2nd version

 

Materials and tools

         5mm diameter plastic straws and 6mm diameter plastic straws

         A ruler

         A (good)pair of scissors

         Proper chemistry text book

 

Introduction

    Straw molecular model is an educational material sample which is affordable and helpful to understand skeleton structure of molecules.

    Each atom is represented by one straw or a combination of several straws and these straws are connected with connector straws (joint) in order to make a molecular model.  For example when you make a carbon dioxide model, first of all you should combine 4 pale green straws which represent four connectable arms of a carbon atom.  Then cut out two straws which have flexible part in the middle of them.  Fold them parallel then they represent oxygen atoms which have two connective arms.  Connect one oxygen straw and carbon with two connectors after that connect another oxygen and carbon again.  You can see straw carbon dioxide molecular model.

    General commercial molecular models have plastic spheres which represent atoms.  But for this straw model it is impossible, therefore please imagine that the carbon atom exists in the middle of the crossing of pale green straws and flexible parts of the purple straws are the centres of oxygen atoms.

    Nevertheless this low cost straw model can show double and triple bonds as well, so we can use this material from upper basic school level to secondary school level.

Covalent radii

    When making molecular model it is important to express relative distance between atoms as accurate as possible.  This explanation sheet basically uses covalent radii (table 1) as distances between atoms.  Due to the types of the straws we can buy, I multiplied 6.4x108 or 4.4x108 on each covalent radius and made length of straws for big models and small models.  If you want to make much accurate models, I recommend using the bond lengths shown on table 2 and adjust lengths of straws.

 

Table 1. Some Covalent radii

Element

H

C

N

O

F

Cl

Br

I

Covalent radius (pm=10-12m)

37

77

74

73

71

99

114

133

Length of straw

(mm)

big model

24

49

48

47

46

64

74

86

small model

16

34

32

32

31

44

50

59

 

Table 2. Bond length

Bond

Comment

Bond length

(pm=10-12m)

Length of straw for one atom (mm)

big model

small model

H-H

In H2

74

24

16

H-Cl

In HCl

127

*

*

H-O

In H2O

96

*

*

Cl-Cl

In Cl2

199

64

44

Br-Br

In Br2

228

73

50

I-I

In I2

267

85

59

N-N

In N2H4

147

47

32

NN

In N2

110

35

24

O-O

In H2O2

128

41

28

O=O

In O2

121

39

27

C-H

Organics

108

*

*

C-Cl

Halogenoalkane

177

*

*

C-O

Organic OH groups

143

*

*

C=O

Aldehydes and ketones

122

*

*

C-C

Organics

154

49

34

C=C

In C2H4

134

43

29

CC

In C2H2

121

39

27

C-C,C=C

(benzene)

In benzene C6H*2

140

45

31

C-N

Amines

147

*

*

CN

Nitriles

116

*

*

 

* adjust the length properly (by cutting the straws or making overlapping)

*2 check "how to make a benzene model"


Colour of straws and atoms and their number of connective arms (valency)

    For the samples of molecular models shown below the following straws were used: pale green straws as carbon (C) atoms, orange straws as hydrogen (H) atoms, purple straws as oxygen (O) atoms and yellow straws as nitrogen (N) atoms.  I used white or pale colour straws as connector straws.  A combination of elements, number of connective arms and colours of straw are shown below.

 

Table 4. Combinations of straw and element

Element

H

C

N

O

number of arm(s)

1

4

3

2

colour of straw

orange

pale green

yellow

purple

Photo














How to make a connector (joint)

    Cut out 40mm (or 30mm for small size models) white or pale colour straw piece.  Fold the piece and make a cut like the folded side picture above.  Straighten the straw and make short cut on both ends which are orthogonal to previous slits.  You can connect open straw ends with these connectors (joints) without any glue or adhesive tapes.

 

Samples of small molecules

H2 H2O O2 N2 NH3


Example 1: Making a carbon (C) atom and a methane model

Follow the illustrated instruction in order to assemble carbon atom model which has sp3 hybrid orbital



Fig. 12

 

Finished sp3 carbon atom model

Fig. 13

 

Methane model (CH4)

 

    If you want to make a methane model, prepare four straw pieces (24mm long pieces for big models, 16mm long pieces for small models) which represent hydrogen atoms and four connectors (joints).

 

    You can also connect carbon atom models. The following figures show how to make an ethane model.

Fig. 14

 

 

Fig. 15

 

Ethane model (CH3-CH3)

 


Example 2: Application of the methane model (Alkane CnH2n+2 & Cycloalkane CnH2n)

Propane (C3H8, CH3-CH2-CH3)

Butane (C4H10, CH3-CH2-CH2-CH3)

 

Cyclohexane (C6H12) Boat

Cyclohexane (C6H12) Chair

 

Example 3: Alkene CnH2n

When you make alkene and alkyne models, please do not apply super glue on the conjunction of C straws!

Ethylene (C2H4, CH2=CH2)

Propylene (C3H6, CH3-CH=CH2)

 

Example 4: Alkyne CnH2n-2

Acetylene (Ethyne)  (C2H2, CHCH)

Propyne (C3H4, H3C-CCH)

Example 5: Alcohol R-OH

Methyl alcohol (CH3OH)

Ethyl alcohol (CH3CH2OH)

 

Example 6: Ether R-O-R’

Diethyl ether (CH3CH2OCH2CH3)

 

Example 7: Aldehyde R-CHO

Formaldehyde (HCHO)

Acetaldehyde (CH3CHO)

 

Example 8: Ketone R-CO-R’

Acetone (CH3COCH3)

Ethyl methyl ketone (CH3CH2COCH3)

 

Example 9: Carboxylic acid R-COOH

Formic acid (HCOOH)

Acetic acid (CH3COOH)

 

Example 10: Ester R-COO-R’

Carboxylic acid + Alcohol

example: Acetic acid (CH3COOH) + Ethyl alcohol (CH3CH2OH)

Ethyl acetate (CH3COOCH2CH3)+ Water (H2O)

 


Example 11: How to make a benzene model

1  Prepare the following straws

Atom

Related

Bond

Actual

length

(pm =10-12m)

Length of straw

for a big model

(mm)

Length of straw

for a small model

(mm)

Number

of straws

C

C-C bond

140

90+20+20

62+20+20

6

C-H bond

110

45+20

31+20

6

H

 

 

24

16

6

Connector

40

26

6

 

Each carbon atom of benzene molecule is connected by sp2 hybrid orbital, therefore carbon atoms of this benzene model have three connective arms.

 

2

3

4

     Make two transversal slit on C-C bond straw (below) on each opposite side and one transversal slit on C-H bond straw.  Make a longitudinal slit on each 20mm part of the straw.

 

     Insert a slit side of a C-H bond straw into a C-C bond straw’s

     Assemble straws along pictures 4-9.

5

6

7

 

8

9

10

     Half of the carbon skeleton of benzene model is finished.

     Finished carbon skeleton of benzene model.

     Insert connectors into C-H bond straws. After that put H atom straws to the connectors.

 


Example 12: Application of the benzene model

Naphthalene

Benzoic Acid C6H5COOH

 

Phenol C6H5OH

Salicylic Acid C6H4(OH)(COOH))

 

Toluene C6H5CH3

 

o-Cresol C6H4(CH3)OH

Please apply these straw molecular models to daily classroom lessons.

m-Xylene C6H4(CH3)2

 

 

 “Straw molecular model” copy right JICA Senior Volunteer Hideo Nakano (Oct 2005, revised August 2008)

inserted by FC2 system