Unit One: Structure of the Atom

Lesson 5: The Quantum mechanical Model

• Comptencies
• Starter Activities
• Main Activites

Competencies 

• Describe the quantum mechanical model
• Describe the main energy levels and sub-energy levels

Material:

1. chart showing main energy level, sub-energy level, and maximum electron capacity ( table 1.5 on page 27 of student textbook)
2. chart for sub-shell, number of orbital and maximum number of electrons

Starter Activity (8minutes)

You can start the lesson by revising Bohr’s electron configuration of main energy levels. Ask students:

• How are electrons arranged in an atom?
• What do we call the stable orbits in which electrons exist?
• What are the names of different energy levels?
• What are the maximum electron capacities of the energy levels? How do you determine the maximum number of electrons   that can be accommodated by a given energy level?
• Give them some time after each question so that they share idea with their partners. Let them raise hands and respond. Write the core idea of the students’ answers on the chalk board. Allow the other students to comment the answers and correct if necessary. . This will give you the opportunity to assess students understanding vis avis the most recent lessons. Don’t forget encouraging slow learners to involve during such revision discussions.

Obviously students can answer most of the questions because they learned it in their grade 7 as well as in the previous lesson.

However, make sure that all students come to the consensus that according to Bohr:

• Electrons in an atom are arranged in restricted number of stable orbits or energy levels in which they neither absorb nor emit energy.
• The orbits are called energy levels or shell
• The energy levels are denoted by K, L, M, N....shells or by principal quantum number  1, 2, 3, 4....energy levels
• The energy levels can accommodate maximum of 2, 8, 18, 32 electrons respectively. The maximum electron capacity of an energy level is calculated by 2n2 where n is principal quantum number.

Evaluation

Give them elements with atomic numbers of 11, 15 and 18 and ask them to write Bohr’s diagrammatic representation of the atoms.

Main Activities

You can use the following analogy to explain the quantum mechanical model. As you have residence address electrons in an atom also have systematic locations. As you can locate your address starting from Kebele to the bed on which you sleep, it is possible to determine position of an electron starting from atom to the volume of space it occupies.

Now you can make students discuss with their partners by raising some questions.

• Are all the kebeles equal in land size or in number of residents? How about the atoms?
• How many houses are there in a kebele? Different or the same? Can you tell number of shells in an atom?
• Do all houses have the same number of rooms? What about the number of sub-energy level in the energy levels?
• You may explain what a sub-shell is at this point. Those main energy levels are sub divided in to sub energy levels. There are four types of sub-energy levels called the s, p, d and f.
• How about the number of beds in a room? Is it analogous to the number of orbitals in a sub-shell?
• Orbital should also be introduced here. It is a volume of space occupied by maximum of 2 electrons.
• The maximum number of people sleeping on a bed under normal condition? Maximum electron capacity of an orbital?
• Students should be informed that the two electrons in an orbital spin in opposite direction.

Then use the chart to harmonize the discussion. As kebeles have different size so do the atoms. Small atoms have lower number of shells as small kebeles have lower number of houses.

Similarly, as houses of many family members, should have normally many rooms or sleeping places, shells or main energy levels of many electrons have many sub-shells or sub-energy levels as well.

Indeed as large rooms accommodate many beds and many people, larger sub-shells have many orbitals and hold many electrons.

Usually two persons (male and female) sleep in a bed.  Orbitals are also safe heavens for two electrons but can never hold more than two electrons. You may tell them humorously that when two persons sleep in a narrow bed, they sleep in opposite direction so that they can fit to the bed. Similarly, electrons always orient in opposite spin in an orbital.

Use the second chart to explain the sub-shell, number of orbitals and their maximum capacity.

Evaluation

You may give them the following individual work.

• Write down the first four main energy levels and their respective sub- energy levels
• Write down the sub-energy levels and their respective number of orbitals.
• What is the maximum number of electrons in each of the four sub energy levels?

You may observe the works of each student and assist those who have difficulties.
Give opportunity for the students to ask questions. Let other students answer the forwarded questions.

Concluding Activity (8 minutes)

Explain how the quantum mechanical model modified the Bohr model?
Make sure that the slow learners are included in the lesson. Let them  respond to this question as follows:

• The main energy levels have sub-energy levels.
• The sub-energy levels consist of a number of orbitals that increases as quantum number increases
• The orbital can accommodate a maximum of two electrons
• The two electrons in an orbital are oriented in opposite spin.