Prime and composite numbers


In this lesson, we'll define, compare and contrast prime and composite numbers.


What are prime numbers?


A prime number is a whole number greater than one which cannot be divided evenly by any other positive whole number except one or itself.


List of prime numbers


We can't list them all here, but let's consider the first few prime numbers


2      3      5      7      11      13      17      19      23      29      ...


Check out a couple of these numbers to make sure they fit the prime numbers definition.


For example, 3 is a whole number greater than one which cannot be divided evenly by any other positive whole number except 1 and 3 - one and itself.


Similarly, 17's a whole number greater than one, evenly divisible only by the positive whole numbers 1 and 17 - again, one and itself.


You may find a prime numbers chart useful


Here's a chart of the numbers 1 through 100 with the prime numbers enlarged/highlighted in bold


1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

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100


Notice how all the prime numbers end in 1, 3, 7 or 9, apart from 2 and 5.


What are composite numbers?


A composite number is any whole number greater than one which is not a prime number. You can evenly divide a composite number by one, itself and at least one other positive whole number.


For instance

  • 4 is a whole number greater than one, evenly divisible by 1, 4 and 2
  • 6 is a whole number greater than one, evenly divisible by 1, 6, 2 and 3
  • 8 is a whole number greater than one, evenly divisible by 1, 8, 2 and 4


Look back at the chart above and notice how all the figures not enlarged/highlighted represent the composite numbers to 100 (with the exception of 1 which is neither prime nor composite).


A quick reminder of how prime and composite numbers differ


Are you now confident you can tell the difference between prime and composite numbers?


Okay, it's time to move on - next up prime factorization.


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