Getting inside the atom

Yasmin Jayathirtha

The concept of the mole is one of the hardest ideas in chemistry. It is closely followed by the structure of the atom. These are difficult partly because they are abstractions and partly because there are very few experiments that can be done easily or demonstrated. Most of the experiments have to be ‘thought’ ones.

The elucidation of the structure of the atom is best told as a story, replete with historical details and posters and maybe little animated videos.

I will outline the story I tell my classes, in the next article. But before telling the story, there are experiments that the students can do. These show the observations that led to the experiments and models and highlight the questions that scientists came up with which they tried to answer by doing other experiments. It will also help the students visualise experiments and come up with deductions. This leads to an understanding of how the models were built up, not just made up out of thin air!

Dalton concieved of atoms as hard spheres, with specific masses and specific combining powers called valencies. What was not known was how these properties arose from the ‘stuff’ that atoms were made up of, and what the stuff itself was. However, there were observations that needed to be explained:

1. The origin of electricity
This can be shown by experiments that all students would have played with; a comb rubbed against one’s hair attracting pieces of paper. The more sophisticated version of this is a stream of water being attracted by a rubbed balloon. Run a thin stream of water from a tap, rub a blown balloon with a piece of nylon cloth and bring the balloon close to the stream. What makes it move? Point out that while people knew the phenomenon, they had no explanation for it.

2. The relationship between electricity and chemical reactions
Electricity causes chemical reactions to happen and chemical reactions generate electricity. This needs an explanation in terms of what the materials are made up of.

A conductivity meter can be used to show that certain substances conduct. Use a conductivity meter to check out a piece of metal, a piece of plastic, kerosene and salt solution. Some conduct and others do not and we have seen earlier that materials can be grouped according to these properties.

Connect graphite electrodes (pencil leads) to a battery connector with crocodile clips, connect a 9V battery and dip the electrodes into a solution of potassium iodide. Show that bubbles come out at one electrode and iodine is formed at the other. When quantitative measurements were made, it turned out that the masses of elements formed were in relation to their atomic masses.

3. Periodicity
Periodicity of properties has to be accounted for in any model of the structure of the atom.

4. Flame tests
Any element that shows a colour in a flame, shows the same colour even in different compounds.

Take copper sulfate, calcium chloride, barium chloride, potassium chloride, boric acid, lithium chloride and strontium chloride. The first five are easily available in most labs and can be bought fairly easily from hardware stores and pharmacies. Light a bunsen burner. If a gas connection is not available in the lab, a kitchen gas stove will work as well. Dip a piece of nichrome wire (heating coils are nichrome and old toasters, irons can be cannibalized) in a wet sample of the salt and hold it to the flame. Try to get as many samples of the salts of the metals, e.g., copper wire can have copper carbonate, hard water or chuna has calcium, iodised salt has potassium and so on. Check the flame colours and point out to the students that sodium vapour lamps give out a yellow colour.

Any model of the structure of the atom has to account for all these phenomena and when you tell the story you can refer to these activities.

Reference: Teacher Plus, August 2010, Elucidating electrochemistry.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.