Unit One: Structure of the Atom

Lesson 2: Discovery of the Electron

• Comptencies
• Starter Activities
• Main Activites

Competencies:

• Explain the discovery of electron
• Describe properties of electron

Materials

•  Charts  with discharge tubes

Starter Activity (6 minutes)

Revision of the modern atomic theory Let the student’s state postulates of modern atomic theory focusing on its basic difference from Dalton’s atomic theory. Make sure that slow learners have understood that atoms are divisible. This can be done by asking them to state and interpret the postulates. You may write the important point of the postulates they mention on the black board. Students should be invited to amend the ideas forwarded by their friends.

Evaluation:

• Which proposals in the Dalton’s theory are modified in the modern atomic theory?
• What were the evidences that served as the basis for the amendments of Dalton’s theory?

Main Activities (28 minutes)

Ask students to predict how scientists came to know the presence of particles in an atom. Appreciate any idea they may forward. This may give you some information about students’ preconception regarding atoms and their particles.

Then give a brief summary of the famous experiment carried out by Thomson using the discharge tube diagram. This could include the following points:

• The apparatus consisted of two electrodes in a tube from which most of the air had been removed

When a high potential difference was applied across the electrodes a stream of particles passed between them

• Thomson called these particles cathode rays because they came from the cathode
• The cathode rays moved straight to the anode, and through a hole in it, to a screen coated with zinc sulphide
• The zinc sulphide glowed when struck by the cathode rays
• The cathode rays were deflected by both magnetic and electrical fields
• The cathode rays were attracted by the +ve electric plate but repelled by the –ve electric plate
• Thomson obtained the same results when he changed the gas and the other materials in the tube
• A cathode ray consists of a stream of electrons

Make students into groups depending on your class size. You may give the following group activities. The activities may be written on a piece of paper in copies to distribute to groups or on a chart and posted on the wall for all students to see or on the chalk board.

Activities

Discuss the following and give reasons

1.  Observe the experimental setup carefully; tell the parts of the apparatus and their functions.
2. When a light paddle was kept on the path of the cathode ray, the paddle was rotated. Tell the reason. An obstacle of a cross kept on the path of the ray casted a shadow on the screen of the discharge tube. Discuss why.

   

3. The rays are attracted to the positive terminal of the electric plate while repelled from the negative terminal. Why does this happen?
4. Thomson observed the same result when he used different gases and materials.

Generalize as in the following table

Let the group representatives come in front to the chalk board and write their findings. The class should comment on the responses of each group.

Evaluation

Ask students to state all the observations and explain their findings
Give the students opportunity to ask questions.
Let other students respond to the posed questions.
You may comment on the answer if necessary

Concluding Activity (7 minutes)

Ask some students randomly to give summary of the day’s lesson. Make sure that students are able to explain that:

• Cathode rays move in straight line because the obstacle on their path casts a shadow on the screen
• Cathode rays have masses because the paddle was rotated when it was strike by the ray.
• Cathode rays are negatively charged because they were attracted to the positive terminal of an electric field
• Cathode rays are contained in all substances because when different materials were used in the discharge tube, they showed the same result.

Students should discuss how, from this evidence, Thomson was able to state that:

• An electron carries a negative charge.
• An electron is a fundamental constituent of all matter