Prime Numbers
A prime number has exactly two factors — 1 and itself — making it a special building block of all whole numbers.
What You Need to Know
Key Concept Diagram
A prime number has exactly two factors: 1 and the number itself
2 is the only even prime number; all other even numbers have more than two factors
The first ten primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
A composite number has more than two factors and can be broken into prime factors
Key Vocabulary
Prime number
A whole number greater than 1 with exactly two factors: 1 and itself
Composite number
A whole number greater than 1 that has more than two factors
Factors
Whole numbers that divide exactly into a given number
Sieve of Eratosthenes
An ancient method for finding all prime numbers up to a given limit by crossing out multiples
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which number is a prime number?
Question 2
Why is 1 NOT a prime number?
Question 3
Which of these is the smallest prime number?
Key Concepts Summary
- ●A prime number has exactly two factors: 1 and the number itself
- ●2 is the only even prime number; all other even numbers have more than two factors
- ●The first ten primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29
- ●A composite number has more than two factors and can be broken into prime factors