Unit One: Structure of the Atom

Lesson 1: The Dalton’s Atomic Theory

• Comptencies
• Starter Activities
• Main Activites

Competencies:

• Describe the existence of small particles of substances
• Describe Dalton’s atomic theory
• Comment on the postulates of Dalton’s atomic theory

Materials: iron wool, iron filings, Paper clips

Starter Activities (10min)

The aim of the activity is to enable students understand how small the atom is and how scientists found its existence.

Make groups of students depending on your class situation. Let them discuss the following issues one after the other.

• Is matter continuous or discrete? Meaning if you start cutting a piece of iron wool, is the process of cutting continuous or stops at a small particle that you cannot cut further?
• Take a single particle of iron filings and try to divide it. Is it possible to do so? How would the particle of iron filings compared with the smallest possible iron?
• How do you think scientists found the existence of the small particle of a substance?
• When elements combine, their mass ratio is always in small whole number. Why do you think is this always true?

Let each of the group representatives make report on one of the issues. The other students should be given an opportunity to comment on the reports of the groups.

Since students have some idea regarding an atom from their primary school chemistry, they may come to a conclusion that matter is discrete. They may remember the ideas of the Greek philosophers, Aristotle and Democritus.

However, you should be careful to avoid misconceptions such as ‘particle of an iron filings is the smallest particle’ because they cannot divide it further.

After they reflected their observation on the second question, you may give them the sizes of iron dust which is 4- 20µm (1m = 1,000,000 µm or 106 µm) and size of an atom of iron to be 156pm (1m = 1000000000000pm or 1012pm). So, size of the atom of iron to its dust particle is 156pm/4x 106pm = 3.9x10-5 or 156pm/ 20x106pm = 7.8x106. This means one dust particle of iron should be broken into 39million to 7.8 million places to find an iron atom.

For the third question, students may suggest that scientists used microscopes to detect the atoms. It should be clear to the students that any instrument with the highest possible magnifying capacity cannot show the atom.

The fourth point is about the finding of Joseph Proust. Whatever idea the students forward, make clear that whole number ratio results only if the small particles of the combining substances unite in whole number; not in fractional parts. This can be explained by analogy. To make a bicycle, one bicycle body is assembled with two tyres. Half bicycle body and two tyres or one bicycle body and one and half tyre etc cannot make a bicycle. Similarly for example, two atoms of hydrogen and one atom of oxygen make one water molecule no more any less. So, whatever pure sample of water you take, it always contains 2:1 ratio of atoms of hydrogen and oxygen which makes 1:8 mass ratio.

With this explanation students understand that the existence of the small particle of a substance is confirmed by experimental processes.

Evaluation

Ask students whether matter is continuous or discrete and how is the existence of atoms proved.

Main Activities (25min)

Introduce the postulates of Dalton’s atomic theory. After making the ideas in the theory clear, allow them discuss each statement of the Dalton’s atomic theory and determine whether they are still valid today.

• Elements are made of small particles called atoms
• Atoms are indivisible and can neither be created nor destroyed
• All atoms of the same element are identical and have the same mass and size
• Atoms of different elements have different masses and size
• Atoms combine in small whole numbers to form compounds

Let their discussion focus on the following two points.

1. Are atoms indivisible particles as stated in postulate 2
2. Are all atoms of an element exactly identical in all respects including mass, postulate 3?

Let some students respond to the activity questions from their previous understanding of the atom. Much is not expected from them because the idea will be clear in the next lesson. However, you should make them curious about these two incorrect postulates by informing them about the findings of the experimental results. That atom is subdivided into electrons, protons and neutrons. Similarly, it is found that there are different atoms of the same element known as isotopes.

Evaluation

You can ask students to state the postulates of Dalton’s atomic theory and to identify which of the postulates are not accepted at the present.

Concluding activity

Procedure

• Show the students a chain of about 10 or more paper clips.
• Ask them what would be the smallest possible particle if you separate the chain.
• If you cut the last paper clip, can it play the role of a paper clip?

Let the students conclude that as the paper clip is the smallest particle of the chain, an atom is the smallest particle of an element.