Prime Numbers

Stephanie Huston, co-op Cheryl Pittore, 6th grade


Description:  We will be reviewing prime numbers in order to prepare students for a unit on fractions.


Rationale:  Students need to be able to break a number into primes in order to find the LCM.  This will help them when it comes time to add and subtract fractions.


Prior Knowledge:  Students have been multiplying and dividing since third grade.  They have a working knowledge of their multiplication tables and should be able to recite multiplication tables from memory.  This will help in the progression to divisibility.  Also, students know the definition of prime numbers and have worked with them in the past.  This should be a review.



4.1 Number & Numerical Operations

4.3 Patterns & Algebra

4.4 Data Analysis, Probability, and Discrete Mathematics

4.5 Mathematical Processes


Objectives:  SWBAT identify a number as prime or composite.

           SWBAT factor a composite number into primes using factor trees.

           SWBAT write the prime factorization of a composite number using exponents


Hook:  Review homework on divisibility.  Have students work with a partner to complete the grid on pg 270.  They should complete the table shading 1-20 blocks.  When finished, make a list of all of the numbers for which you could only make one rectangle.  What can you conclude about the numbers that can be made into only one rectangle?



1. Explain to the class that they now have a list of the prime numbers from 2-20.  Hand  out the grid of numbers 0-99.

2. Explain to the class that they will now be identifying all of the prime numbers from 1-200.  Using a poster size grid, we will complete the chart as a class.  The result is that all of the composite numbers are shaded, and the prime numbers remain blank.

3. Now that they have identified what a prime number is, and we know our divisibility rules, we will be making factor trees for a variety of composite numbers.

4. As a class, we will find the prime factorization for 630.  We will create a student guided factor tree on the chalkboard.

5. As the tree branches out, we will circle the prime numbers.  When we have gotten to all prime numbers, we will express the prime factorization using exponents.

6. With a partner students will complete the prime factorization for pg 271 #1-5

7. Review #1-5, and answer any questions that students may have.


Assessment:  I will walk around the room and look as the students work to see if they understand what is going on.  If I see something is still unclear, I will put it into other words to make it more understandable.  In addition, students will be given a HW assignment to test their understanding of the class work on prime factorization.


Prime Number - any number greater than 1 that has exactly 2 factors, 1 and itself.


In the chart, the yellow numbers are prime and the numbers with an X over them are composite (more than 2 factors).


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