Unit One: Structure of the Atom

Lesson 3: Atomic number, mass number, Atomic mass and Isotope

• Comptencies
• Starter Activities
• Main Activites

Competencies 

• Tell the number of protons and electrons in an atom from the atomic number of the element
• Determine the number of neutrons from given values of atomic numbers and mass numbers
• Explain the atomic mass and isotope

Material

Work sheet

Note for the teacher

This lesson may take two or more periods. The first period should be used to introduce by giving starter activity and explaining the procedure of the activity. All the groups and the individual students should know their tasks during the activity. You should supervise the groups during their discussion and make clear any misunderstanding they may have. Students should be free to ask any question at any time and they should be encouraged to actively participate because they explain the tasks to their friends later on.

Starter activities (5 minute)

You may start the lesson by the analogy of the assignment of roll number to the students in a classroom and its importance. Let them discuss how a roll number is given to a student and its importance. Guide them to come to the conclusion that roll number is assigned on the basis of alphabetical order of name of the students and teachers use them as short hands for the name of the students.

Now you can ask them how we identify different atoms of the same element and   atoms of different elements using similar procedure.
Then introduce the lesson

Evaluation

You may ask them: can two or more students in a class have the same roll number?

Main activity (25 minutes)

Procedure

• Group your students into a maximum of 8 students taking slow, medium and fast learners into consideration. This is a base group. Let the students in each base group elect their chair person and secretary.
• Give numbers 1 to 4 to the students in a group. This will be repeated in a group if the number of the students is more than 4. Meaning there may be two 1’s and two 2’s etc.
• Let all the 1’s make one group, all the 2’s the second group, the 3’s third group and etc. These groups are called expert groups. They should also have a chair person.
• Assign each expert group a section of the textbook on atomic structure
• Group 1- atomic number, textbook page 15
• Group 2- mass number, textbook page 15-16
• Group 3- atomic mass, textbook page 17-20
• Group 4- isotopes, textbook page 17-20
• Provide a focus point of discussion on a chart or write it on a chalk board. The focus sheet will contain questions that help the students select key points/ideas from the reading. Points of discussion for each group are:

Atomic number group

1. What is an atomic number?
2. How is it related to the numbers of electrons and protons in an atom?
3. What is the purpose of an atomic number?
4. Can two or more different atoms have the same atomic numbers?
5. How is an atomic number used in the periodic table?

Mass number group

1. What is mass number?
2. How do you determine it from the number of nuclear particles?
3. Is mass number the same for all atoms of the same element? why?
4. What is the relationship between the number of the protons, the number of neutrons and the mass number?
5. How do we use mass number to identify a particular atom of an element?

Isotope group

1. What is an isotope?
2. What is the difference between isotopes of the same element?
3. Give examples of elements with isotopes.
4. Represent isotopes of the same element using symbolic representation

Atomic mass group

1. How do you compare the actual mass of an atom to the mass of objects in your environment?
2. Is it possible to measure mass of an atom using ordinary scale?
3. What did the scientists do to avoid the difficulty of using the extremely small mass of the atoms?
4. What is relative atomic mass?
5. What is the standard element in the establishment of relative atomic mass? Which element is used for this purpose?
6. What is the unit of relative atomic mass? How is it determined?
7. What is the average atomic mass?
8. How is the average atomic mass determined?
9. Calculate the average atomic mass of element X. It has two isotopes X-40 and X-41 with relative abundance of 60% and 40% respectively.

• Instruct each group to read their section of the textbook and answer the questions on the focus sheet.
• Students in the expert groups will discuss the reading and come to an agreement on the key points of the text.  Students will revise the answers on their focus sheets, if necessary.
• Instruct students to return to their base groups and take turns to share the key points from their section of the text.  Students will answer the questions on the focus sheet as their group members discuss the key points.

Evaluation

Let the base group representatives report the key points of their discussion to the whole class. You may ask them the main key points of each topic.

Concluding activity

Let the students perform the following activities.

• Given numbers of protons, neutrons and electrons, let them write symbolic representation of the element as AZX.
• Give number of protons, number of neutrons or mass number, let them calculate the other.
• Let them define relative atomic mass and average atomic mass.
• Given relative abundance and isotopic masses let them calculate average atomic masses.