Lesson Plan on Half-life ( A Lesson Intended for Fourth Year High School)

Lesson Plan on Half-life

( A Lesson Intended for Fourth Year High School)

I. Objectives

At the end of the period, the students are expected to:

a. define half life;

b. solve problems regarding half-life; and

c. participate actively in class discussion and activities.

II. Subject matter

Half-life

Reference: Physics (7^th edition) by Cutnell and Johnson

                        Chemistry the Central Science (9^th edition) by Brown, Lemay and Bursten

a. Concepts:

ü Half-life (T_½) is the time required for half of any given quantity of a substance to react.

ü The rates of decay of nuclei are commonly discussed in terms of their half-lives.

ü Each isotope has its own characteristic half-life.

b. Strategy: Lecture

c. Materials: chalk board, manila paper

III. Procedure

A.     Preliminaries
1. Opening prayer

Teacher 1: Everybody stand-up, RJ please lead the prayer.

RJ: Our father who art in heaven… Amen

Teacher 1: Please take your seat.

2. Checking of attendance

                        Teacher 1: Please be on your proper seat.

(The teacher will check the attendance through the seat plan.)

B.     Initiatory activities

1.      Review of previous lesson (Topic: Radioactive decay series)

Teacher 1: Class, what was our lesson last meeting?

Marlon: Our topic last meeting Ma’am is about Radioactive decay series.

Teacher 1: Very good, thank you Marlon. So, in line with that, I have here a diagram of a radioactive decay series of ²³⁸U to ²⁰⁶Pb. As you can see, there are gaps in the series. To complete the series, you are going to fill in that gap by putting these pieces of papers cut in circle into their proper places. I will give you 1 minute to spot the proper location. As soon as you have decided, you can go to the board and answer.

Teacher 1: Ok class, very good. You have already mastered our previous lesson. Before we proceed to our topic for today, is there anyone who can read our objectives?

Eden: At the end of the period, the students are expected to:

a.       define half life;

b.      solve problems regarding half-life; and

c.       participate actively in class discussion and activities.

Teacher 1: Thank you. Those are your goals today. First, you are going to give your own definition of half-life based on our discussion. Next, we will give you problems related to it and you will solve them. During our discussion, we are expecting you to be participative and active in our activities.

2.      Motivation

The teacher will present to the whole class three pictures of Helium atoms with different amounts. The first picture contains ten atoms of Helium, the second contains five and the third contains two and a half.

Teacher 1: I have here pictures of Helium atoms. I want you to observe the number of atoms in each picture arranged in descending manner. I will give you 30 second to do so. Afterwards I will ask some of you to tell us what have you observe in the pictures. Your 30 seconds starts now.

After 30 seconds…………….

Teacher 1: ok class your time is up. Anybody to share his/her observations?

Hasnia: I have observed that the pictures contain atoms of Helium which are intact with one another.

Teacher 1: Very good Hasnia. The pictures contain Helium atoms tightly held with each. Any other observations?

Jeorge: The three pictures differ in the number of atoms present in it. As I have observed the number decreases into half of its original number.

Teacher 1: Very good Jeorge. The pictures differ in the number of Helium atom present in it. It decreases into half of its original number. From ten it becomes five and again decreases to its half which is two and a half. The decrease of atom into half of its original value would be our topic for this afternoon .That is what we call Half-life on an atom.

C.     Lesson Proper

Teacher 2: With the pictures I have just presented to you, we have partially defined what a half-life is. To further understand it, I have posted on the board the definition, graphical representation and sample problem related to the topic. Is there anyone who wants to volunteer to read the definition?

Rusty: Half-life (T_½) is the time required for half of any given quantity of a substance to react.

Teacher 2:  Thank you Rusty. Half- life is the amount of time needed so that half of a given element will react. An element, when it reaches its half-life decreases its amount into half of its original value. As you can see in the graph, as the time increases, the amount of the given element decreases. It only tells us that the amount of the element and the required time for it to decay is inversely proportional.

(The teacher continue the discussion)

Application: Group activity – solving

Teacher 2: I have here problems for you to be solved as an application of our lesson. Group yourselves into five and work as a team in answering these. I will give you 5 minutes to work on this. After that, I will choose one representative from your group to present and explain your answer. You can write it in any sheet of paper. (The will now post the problems on the board.)

1.      The amount of a radioactive sample at the start of an experiment is 500 g. The amount present after 12 days is 125 g. What is the half-life (in days) of the sample? (6)

2.      How much time is required for a 5.75 mg sample of ⁵¹Cr to decay to 1.44 mg if it has a half-life of 27.8 days? (55.6)

D.     Formative Assessment

Short Quiz (15 points)

Teacher 1: Since you now know how to solve problems regarding half-life, I think you are now ready for a quiz. Kindly get one-half sheet of intermediate paper and answer the following question. Please work individually. (The teacher will post questions on the board.)

1.      Define half-life. (5 points)

2.      Problem Solving: (10 points)

The half-life of tritium (hydrogen-3) is 12.3 yr. If 48.0 mg of tritium is released from a nuclear power plant during the course of an accident, what mass of this nuclide will remain after 12.3 yr.? After 49.2 yr?

E.     Assignment

Teacher 1: For your assignment, copy and answer this problem in a one-half sheet of intermediate paper to be submitted on our next meeting.

Suppose 3.0 x 10⁷ radon atoms are trapped in a basement at the time the basement is sealed against further entry of the gas. The half-life of radon is 3.83 days. How many radon atoms remain after 31 days?