Structure of atoms is an abstract concept. Teachers have difficulty in conveying the three-dimensional concept accurately to students while many students find it hard to visualise it using conventional diagrams and charts. A clear and accurate understanding of atomic structure is vital in learning many other related topics in Chemistry such as chemical bonding, reactions, and reactivity. Besides, the learning of physical concepts like nuclear reaction also needs a good grasp of structure of atoms.

The use of these atomic models aims to enable students to better visualise and hence better understand the three-dimensional concept of atomic structure through either

i. observing the models, or
ii. building the models themselves in class (or as a project)

 

1. Conventional use of diagrams and charts can clearly convey single dimensional (or the most two dimensions) movements of electrons in an orbit. However, with the use of this model, electrons can be moved along the orbit (made of wire) and the orbit itself can be turned around freely to form a spherical surface; hence the concept of a spherical shell. With the combination of these two movements, an electron can be observed to move in any direction on the surface of a sphere. This helps students correct their misconceptions that an electron orbits around a single-plane orbit or sphere (please see 9-D1).

2. Using this model, positive ions can be illustrated by removing one or more electrons from the outermost shell while negative ions can be made by adding one or more electrons to the outermost shell. Size of the resulting ions can be clearly compared and observed.

3. The content, relative size and density of the nucleus compared to the whole atom is clearly presented.

4. Colours clearly differentiated the sub-atoms and their positions in the atom.

5. The number of sub-atoms can be observed, thus establishing the concept of atomic number and atomic mass.

6. A series of models for different atoms can be built to show differences and similarities between atoms.

i. White polystyrene balls of diameter 1½ cm - 2 cm

ii. Water colour (red and green or other colour of individual's choice)

iii. Steel wire, diameter between ½ mm - 1 mm (steel wire is used because it forms smooth and perfect circle when folded)

iv. Thread, approx. ½mm diameter (e.g. Anchor No 2)

v. Needle (or Constantine wire, SWG 24)

vi. Cellophane tapevii. Pliers to cut steel wire.

[For the purpose of presentation here, a sodium atom, an oxygen molecule, a sodium ion (positive) and an oxygen ion (negative) are selected]

Sodium Atom

1. 11 polystyrene balls are coloured reds using watercolour to represent protons.

2. 12 polystyrene balls are coloured green using watercolour to represent neutrons.

3. 11 white polystyrene balls represent electrons.

4. The red and green polystyrene balls are stringed alternately using a needle and thread, leaving two long ends (9-D2(a)). It is then folded and tied firmly to form a compact nucleus (9-D2(b)).

5. The first electron shell is constructed by poking a suitable length of steel wire through 2 white polystyrene balls. The wire is then folded and taped at the ends to form a round circle (orbit) (9-D2(c )).

6. The second shell of 8 electrons and the third shell of 1 electron are constructed in the same manner but of increasing sizes. (9-D2(d) and 9-D2(e)).

7. Using two longs ends of the thread from the nucleus, the shells are tied in the form of concentric circles (9-D1).

8. One end of the thread is tied to a support (a retort stand is suitable) and the model is ready for use.

Sodium Ion

1. The electron in the outermost shell is removed together with the shell. The remaining structure is a positive sodium ion (9-D3).

2. Reducing an electron (negative charge) while the nucleus remains unchanged gives a good picture of one extra positive charge compared to negative charge in the ion. At the same time, the size is seen to reduce significantly.

Oxygen Ion

1. Using the same method as in constructing a sodium atom, an oxygen atom is constructed using 8 protons (red polystyrene balls), 8 neutrons (green) and 8 electrons (white).

2. 2 additional electrons (white polystyrene balls) are then added to the outermost shell to make a total of 8. The result is 2 more negative charges compared to positive charges.

Oxygen Molecule

1. Two oxygen atoms are constructed and joined at the outermost shell, sharing four electrons (9-D4).

2. Though there are some restrictions of movement of the outermost shells since they are joined together, the idea can be explained and likened to two balls touching together and therefore the electrons are still able to move in all directions.

Method 1: Observation of ready-made atomic models by students

(a) Models of the following are made by teacher beforehand and distributed to 5-10 groups of students during class, each receiving at least one model of an atom:· Atoms with Atomic Number ranging from 1 to 10· Sodium atom and its ion· Oxygen ion and oxygen molecule

(b) Students observe carefully and analyse each model and discuss with teacher the structure of atoms.

(c) Teacher discusses and familiarises the students with the sub-atoms, their respective positions, charges, and how they move in the models.

(d) The students are then asked to observe, identify and count the number of sub-atoms in each atom/ion/molecule given their respective groups. Tabulate their findings on the blackboard.

(e) From their findings, the teacher can then introduce and discuss various other concepts e.g. atomic number, atomic mass, isotopes, molecules, what makes an ion different from atom, reactivity, Periodic Table, and radioactivity.

Method 2 : Construction of atomic models by students

(a) This can be done either as a laboratory activity or as a project. It enhances their learning since it involves not only mental work but also psychomotor, emotional and social activities as well.

(b) Students are divided into groups of 3-4 members. Smaller groups are better as it encourage all members to participate actively.

(c) Each group chooses an atom (or more if the amount of time allows) to be constructed and is given the necessary materials.

(d) The teacher discusses briefly but precisely the procedures with students. Optionally, students can be left to discuss amongst themselves and work out the procedures of their work.

(e) Models are then constructed as described in the previous section with further guidance/discussion with each group.

(f) Discussion is then focussed on comparing and contrasting the models obtained for better understanding.

(g) The models are then displayed in the laboratory for constant reference.