---
title: "Prime numbers"
description: "Know and use the vocabulary of prime numbers, prime factors, and composite numbers; establish whether a number up to 100 is prime; recall prime numbers up to 19"
canonical: https://lightmysky.com/learn/mathematics/prime-numbers-mt_y1XCVsIelg
source: https://lightmysky.com/learn/mathematics/prime-numbers-mt_y1XCVsIelg.md
retrieved: 2026-10-19
---

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# Prime numbers

Know and use the vocabulary of prime numbers, prime factors, and composite numbers; establish whether a number up to 100 is prime; recall prime numbers up to 19

Subject: Mathematics · Area: Multiplication & Division · Ages 9 to 10
Page: https://lightmysky.com/learn/mathematics/prime-numbers-mt_y1XCVsIelg

## Ready when they can

- Explain that a prime number has exactly two factors: 1 and itself
- Determine whether 51 is prime or composite and justify the answer
- List all prime numbers up to 19 from memory

## Lesson: Prime numbers

You already know a factor is a number that divides exactly into another number, with nothing left over. A composite number has three or more factors. A prime number is stricter: it has exactly two factors, 1 and itself. That is why 1 is not prime, it only has one factor, itself.

*(drawing: Six dots make a neat rectangle, 2 rows of 3. That extra factor makes 6 composite. A prime number like 7 can only ever line up in a single row.)*

**Example.** Is 51 prime or composite? Try dividing by small primes. 51 divided by 3 is 17 exactly, with nothing left over. So 51 has the factors 1, 3, 17, and 51, four factors in total. That is more than two, so 51 is composite.

*(drawing: These eight numbers are the only primes up to 19. Try saying them from memory: 2, 3, 5, 7, 11, 13, 17, 19.)*

**Tip.** To test any number up to 100, you only need to try dividing it by 2, 3, 5, and 7. If none of those divide it exactly, the number is prime.

**Recap.** A prime number has exactly two factors, 1 and itself, and you can test any number up to 100 by checking a few small primes.

## Practice

18 questions on this page, each with its working shown.

## Needs first

- [Factors, multiples, and primes](https://lightmysky.com/learn/mathematics/factors-multiples-and-primes-mt_FHIAv6dfhU)
- [Odd or Even](https://lightmysky.com/learn/mathematics/odd-or-even-mt_hniI4E-OCE)

## Opens up

- [Finding a Counterexample](https://lightmysky.com/learn/mathematics/finding-a-counterexample-mt_EoLyw6eP87)
- [Factors, multiples, and primes (age 11+)](https://lightmysky.com/learn/mathematics/factors-multiples-and-primes-age-11-mt_xhoOWnhtHq)