4th Grade Math: PRIME & COMPOSITE NUMBERS
1. What is a prime number? (4th grade students MUST know all primes between 1-100).
2. What is a composite number? (4th grade students MUST be able to identify the factors
of a number).
3. What is a unique number?
1. Prime numbers are natural numbers that have only 2 factors: 1 and itself. An example is
the number 17. 17 has only two factors, 1 and 17. Factors are numbers that divide evenly
into another number. Only 1 and 17 divide evenly into 17 without any remainders, so 17 is a
prime number.
Can you name other prime numbers? Remember, they can only have 2 factors!
Here are the prime numbers between 1 and 100. There are 25 of them. They are organized
by groups of 10.
2,3,5,7
11,13,17,19
23,29
31,37,
41,43,47
53,59
61,67
71,73,79
83,89
97
Notice that they are ALL odd numbers except for 2! Please see the “sticky six” link for
more work with prime numbers.
2. Composite numbers have more than 2 factors. An example of a composite number is 9. 9
has three factors: 1,3,9. All of these factors divide evenly into 9. Another example is 24.
The factors of 24 are: 1,2,3,4,6,8,12,24. These numbers divide evenly into 24. I am not
going to list all of the composite numbers between 1 and 100; they are simply all the
numbers that ARE NOT prime. Please see the divisibility rules link.
3. This leaves us with the number 1. The number 1 is neither prime nor composite. It can’t be
prime because it doesn’t fit the definition of having TWO factors, 1 and itself. It can’t be
composite because it doesn’t have MORE THAN 2 factors. 1 has only ONE factor: 1. We call
1 a UNIQUE number because it doesn’t fit into these two groups.
Check your understanding
Circle the Prime numbers: 19 21 63 71 81 12 2
Hint—there are 4
50 77 97 51
19, 71, 2, 97 Note, 51 is NOT prime (factors are 1,3,17,51)