Unit Two: Periodic classification of the Elements

Lesson 1: The Modern Periodic Law

• Comptencies
• Starter Activities
• Main Activites

Competencies 

• State the modern periodic law
• Describe periods
• Describe group
• Describe  the relationship between the electronic configuration and the structure of the modern periodic table

Materials:

• Bohr model of the atoms of atomic number 1-18 drawn on a hard paper and carefully cut out. The number of models must be equal to the number of groups of students. Name and symbol of the elements should not be written on the model.
• Manila paper for each group, Gum

You can print and carefully cut the figures of the models given at the back of the manual.

Starter Activity (7minutes)

Imagine that students of grades 1-8 in a primary school are lined up straight by a sport teacher during a flag ceremony. Supposing that each grade has only one section, ask students to discuss the following in their groups

• How are the students in the vertical and students in the horizontal rows related?
• Does it resemble the Mendeleev’s periodic table? 
• Explain the drawback in the periodicity of the students’ arrangement?

Let them discuss the relationship between the vertical and the horizontal lines.

Gear students’ discussion to make the following conclusion:

• All the shortest students of the classes are in the first raw and the second most short students are in the second raw etc.
• The grade of the students in a raw consecutively increases by one from 1 to 8
• The height of the students in a vertical column is also increasing gradually.
• The students in a vertical raw are in the same grade.

Although there are certain similarities in the rows as well as in the columns, the weight of students along the horizontal rows may not show some regularity. Analogously, this may be compared with the   Mendeleev’s classification which was based on atomic masses.

Evaluation

With respect to periodicity, ask the students to find out the anomalies of the Mendeleev’s periodic table and that of the arrangement of students in a line during the flag ceremony.
Students should mention there is irregularity in the weight of students along the horizontal rows. This is the same as the irregularity in Mendeleev’s table which was based on atomic masses. There is irregularity in properties because an element of large mass has been arranged in a row before an element of a small mass

Main activities (27 minutes)

Procedure

• Arrange  students into groups equal to the number of  sets of models you prepared
• Provide each group with the models
• Order the students to sort and arrange the models with certain types of similarities into horizontal row and vertical column.
• Remind them that their table should come up with periodicity
• If you observe them facing difficulty during your supervision, give them clue to remember electron configuration of the elements.
• Let them paste the models on the manila with the gum
• In their discussion, they should give reason for their arrangement
• What criteria they used to arrange elements in horizontal rows?
• What criteria they used to arrange elements in vertical columns?
• What generalization can be drawn regarding the classification of the elements?
• The group representative should come in front to present the groups work
• Let the rest of the class comment and discuss  the tables presented

Dear teacher, students might have good back ground knowledge of the  periodic classification from their grade 7 chemistry and they might come up  with correct conclusion. Make sure that they managed the following points

• All elements in a horizontal row have the same number of shells and the horizontal rows are called periods
• Number of shells of atoms of the elements equals the period number of the element in the periodic table
• All elements in the vertical columns have the same number of valence electrons and vertical columns are called groups
• Number of valence electrons of atoms of the elements equals the group number of the element in the periodic table

Periodic properties of the elements are the function of their atomic numbers.
Their table should look like the following but in Bohr’s model.

Evaluation

Ask the students the following questions

• What do elements in the same horizontal rows have in common?
• What do elements in the same vertical columns have in common?
• Are they arranged according to the increasing order of their masses as in Mendeleev’s table or according to the variation of another important factor?
• State a periodic law based on your observation.

Concluding activity

You can use the long form of the periodic table to conclude the lesson. Display the periodic table so that all students can see it.
Let them closely look at the table to compare their works of arranging the elements.
You may give them the following individual activities so that they can come up with generalization.
Complete the following tables.

• Find the valence numbers by writing electron configuration of at least one element from each group.

• Find the numbers of shells by writing electron configuration of at least one element from each period.

Check the performance of student and help the slow learners to come to the conclusion.

Students may do the activities as follows:

• Finding valence number

They may write electron configuration of any one of the group IA elements. For example:

11 Na – electron configuration for the sub- energy levels is: 1s2 2s22p63s1. The last energy level, the L-shell contains only 1 electron. Therefore, the number of valence electron for group IA is 1.

Group IIA: 20Ca can be taken as an example, which has electron configuration of 1s2 2s2 2p6 3s2 3p6 4s2. The last energy level of calcium, the fourth energy level, contains 2 electrons. Therefore the number of valence electrons of group IIA elements is 2. The number of valence electrons of the other groups can also be found in the same way.

• Finding Numbers of shells or energy levels

The number of shells of an element is also determined by writing its electron configuration. The number of shells or energy level of an element is the number of shells its electrons occupy. Any one element of a period can be taken as an example. 15P can be taken from period 3, which has electron configuration of 1s2 2s22p63s23p3. Since electrons of phosphorus occupy 3 energy levels, it has 3 shells. Similarly 31Ga can be taken from period 4. Gallium has an electron configuration of 1s2 2s22p63s23p64s23d104p1, as the result, its number of shells is 4. In this way students can complete the tables and make conclusions that the number of valence electrons is equal to the group number of the element in the periodic table. Similarly the number of shells of an element is equal to the number of period of the periodic table in which it is found.

Evaluation

What is the difference between Mendeleev’s periodic law and the modern periodic law?

Give examples from the periodic table to show that Mendeleev’s classification, which was based on atomic mass, has short comings.

What do we call the horizontal rows? What is the common characteristic of the elements in rows?

What do we call the vertical columns? How are the elements in the columns related?

Make sure that the slow learners correctly respond to the above questions. These are:

Mendeleev’s periodic law is based on atomic mass while the modern periodic law is based on the atomic number. One of the students should come to the periodic table to show the mass order of K and Ar. The others may tell that many elements have isotopes with different masses. Atoms of the same elements cannot have different positions in the periodic table.

The horizontal rows are called periods and they contain elements with the same number of electron shells. The vertical columns are called groups and elements in a group are related because they have the same number of valence electrons. Encourage the fast learner students to work out electron configurations of the heavy elements and find their period and group numbers in the periodic table.