Linking data to atoms

Yasmin Jayathirtha

As we saw in the last article, the periodic table gives students an insight into a qualitative way of classifying types of elements. The table also leads naturally to students getting curious about the numbers associated with the elements. And this can lead naturally to atomic structure and a way of linking the quantitative data from reactions to the world of atoms, molecules and ions. All teachers know that this is the hardest to teach and for the students to understand. There are two ways to tackle this; the first, teach the idea of relative atomic mass (RAM) with activities and when the atomic structure is taught, how the RAM arises becomes apparent. In the other way, teach atomic structure first and link to the RAM.

I prefer the first, for two reasons. It evolved historically and there is an elegance in inferring the data and then having the atomic structure vindicate it. In the second way, we are learning something that others have discovered and have to follow the arguments they have made. The temptation is to teach these concepts using the blackboard and numbers, because it is just arithmetic after all. But that is what makes it hard for the students and at the age these concepts are taught, they find thought experiments difficult. I would say that this is probably why most students say they find chemistry difficult. Rather than put all the activities together, I would like to go from teaching point to teaching point outlining the activities the students can do.

The materials to use: ball bearings of different sizes, seeds, plasticine, coins, sweets (maybe), an electronic balance which your friendly neighbourhood shopkeeper may oblige.

Teaching points
1. Weighing as a means of counting: Take a large number of coins and ask the students to count them. We will usually get differing counts. Ask them to weigh* a fixed number of coins (1 or 2 or 5) and then weigh all the coins together. Calculate the number of coins altogether. Point out that large temples (e.g.; Tirupati) use this method to determine the amount in the donation boxes.

We can buy a large bag of sweets and ask, ‘Will there be enough to give everybody two sweets?’ or ‘How can we estimate how many we can give each member of the class?’ Again weigh a few and then check the weight of the bag and estimate how many there are.

2. Weighing as a means of comparing numbers of different objects:
This is the crux of the chemistry lesson on RAM! We can use different seeds, ball bearing balls of different sizes, marbles or large beads of different sizes.

Take the smallest or the lightest of your object and weigh a sufficient number to give either 1gm or 2gm. Now weigh the same number of the other objects and tabulate their mass. My sample data was like this:

Seed: Fried gram, Number: 20, Mass/g: 2
Seed: Kabuli channa, Number: 20, Mass/g: 12
Seed: Groundnuts, Number: 20, Mass/g: 8
Seed: Rajma, Number: 20, Mass/g: 14

This can be used to give an idea of RAM. Kabuli channa is six times the mass of fried gram and so on. Further practice can happen with pre-made packets as – how many channa seeds in this 24g packet? Or this packet contains 100 sweets, what is its mass? After a few rounds of concrete examples, we can move on to blackboard problems. These experiments give actual masses relative to the lightest ones. To get the idea of even the mass of the lightest one being relative, there is a rather nice activity with plasticine.

Make lumps of plasticine having masses 0.82g, 1.64g, 2.46g and other multiples. After we find out the masses relative to the lightest one, we can point out that rather than using the actual mass, we can say when the lightest is 1, the other is 2, etc. This is the way RAM works and we can point out that if we consider one atom of H (the lightest element) as having a mass of one, then the other elements have masses relative to it, like C=12, O=16, etc. Just as in the seeds example, when 10 fried gram made 1g, we can say a large number, say N or L (usual abbreviations) of H atoms make 1g. This is the introduction to the mole and for the younger students we can stop here. But for the more senior classes, we can move on to give the number and the definition of a mole. We can also point out that the reference has varied from time to time and at present is 12C=12. It can be extended in many ways to cover isotopes and concentrations.

Reference

• Teaching Secondary Chemistry Bob Mcduell editor (John Murray, 2000)

*It is more correct to say ‘get the mass of’, rather than ‘weigh’ but I have used weigh as being more colloquial.

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