What is a prime number?
A prime number is a whole number greater than 1 that has exactly two factors: 1 and the number itself. A number with more than two factors is a composite number.
Is 1 a prime number?
No. The number 1 has only one factor, so it is neither prime nor composite. The number 2 is special — it is the only even prime number, because every other even number can also be divided by 2. All other prime numbers are odd.
Key idea
Prime = exactly 2 factors (1 and itself). To test a number, try dividing it by 2, 3, 5 and 7.
Worked example
Is 9 prime? The factors of 9 are 1, 3 and 9 — three factors — so 9 is not prime. Is 11 prime? Its only factors are 1 and 11, so 11 is prime.
Remember
- Primes under 20: 2, 3, 5, 7, 11, 13, 17, 19.
- 1 is not a prime number.
- 2 is the only even prime.