We say that a number is a prime number if the only factors of that number are 1 and itself. If the number has three or more factors then we say that the number is not a prime number. Such numbers are called composite numbers. The number 1 is by definition considered to be neither a prime number nor a composite number.

**Primes between 1 and 20**

There are a total of 8 prime numbers between 1 and 20. These eight numbers are 2, 3, 5, 7, 11, 13, 17, and 19.

- The number 1 is neither prime nor composite.
- The number 2 is a prime because it has only two factors – 1 and 2.
- The number 3 is a prime because it has only two factors – 1 and 3.
- The number 4 is not prime because it has factors 1, 2 and 4.
- The number 5 is a prime because it has only two factors – 1 and 5.
- The number 6 is not prime because it has factors 1, 2, 3 and 6.
- The number 7 is a prime because it has only two factors – 1 and 7.
- The number 8 is not prime because it has factors 1, 2, 4 and 8.
- The number 9 is not prime because it has factors 1, 3 and 9.
- The number 10 is not prime because it has factors 1, 2, 5 and 10.
- The number 11 is a prime because it has only two factors – 1 and 11.
- The number 12 is not prime because it has factors 1, 2, 3, 4, 6 and 12.
- The number 13 is a prime because it has only two factors – 1 and 13.
- The number 14 is not prime because it has factors 1, 2, 7 and 14.
- The number 15 is not prime because it has factors 1, 3, 5 and 15.
- The number 16 is not prime because it has factors 1, 2, 4, 8 and 16.
- The number 17 is a prime because it has only two factors – 1 and 17.
- The number 18 is not prime because it has factors 1, 2, 3, 6, 9 and 18.
- The number 19 is a prime because it has only two factors – 1 and 19.
- The number 20 is not prime because it has factors 1, 2, 4, 5, 10 and 20.