Prime number
- CBSE · NCERT Class 6, Ch 5 Prime Time
A prime number is a number greater than 1 that has exactly two factors, 1 and itself, such as 2, 3, 5, 7 and 11.
A prime can only be split as 1 times itself. 7 is prime, because only 1 × 7 makes 7. 9 is not prime, because 3 × 3 also makes 9. A number with more than two factors is a composite number.
Two facts are worth keeping in mind. The number 1 is not prime, because it has only one factor. And 2 is the only even prime. Every other even number has 2 as a factor as well.
A small example
Is 91 prime? It is odd and does not end in 5, so 2 and 5 do not divide it. Try 3: 9 + 1 = 10, which is not a multiple of 3. Try 7: 7 × 13 = 91. So 91 has the factors 1, 7, 13 and 91. It is not prime.
The primes below 30 are 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29. The Class 6 chapter finds them with the Sieve of Eratosthenes. You cross out the multiples of each prime in turn, and the primes are left.
Why it matters later
Class 10 Chapter 1 opens with the fundamental theorem of arithmetic. It says every composite number is a product of primes in exactly one way, apart from the order of the primes. A factor tree only works if you can tell a prime when you reach one.
Sources
Facts last checked against these sources on 30 September 2026.
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