Prime and Composite Numbers

Learn to identify prime numbers (divisible only by 1 and themselves) and composite numbers (having more than two factors).

Elementary20 minLesson
Printable worksheet

Definition

A prime number is a whole number greater than 1 that has exactly two factors: 1 and itself.
A composite number is a whole number greater than 1 that has more than two factors.
Examples:
  • 77 is prime because its only factors are 11 and 77
  • 1212 is composite because its factors are 1,2,3,4,6,121, 2, 3, 4, 6, 12
Special case: The number 11 is neither prime nor composite. It has only one factor (itself).

Try it now

Which of these numbers is prime?

Worked Examples

Determine whether 1717 is prime or composite.
1

List what we need to check

We need to find if 1717 has any factors besides 11 and 1717 → Check divisibility
2

Check divisibility by 2

17÷2=8.517 \div 2 = 8.5 (not a whole number) → Not divisible by 2
3

Check divisibility by 3

17÷3=5.67...17 \div 3 = 5.67... (not a whole number) → Not divisible by 3
4

Check divisibility by 4

17÷4=4.2517 \div 4 = 4.25 (not a whole number) → Not divisible by 4
5

Do we need to check more?

5×5=25>175 \times 5 = 25 > 17, so we only need to check up to 4 → No more checks needed
6

Conclude

1717 has no factors other than 11 and 1717 → 1717 is prime

Common Mistakes

Thinking 11 is a prime number

Why it's wrong: A prime number must have exactly two different factors. The number 11 has only one factor (itself).

Correct: 11 is neither prime nor composite. The smallest prime number is 22.

Thinking all odd numbers are prime

Why it's wrong: Many odd numbers have factors besides 11 and themselves.

Correct: 9=3×39 = 3 \times 3, 15=3×515 = 3 \times 5, 21=3×721 = 3 \times 7 are all odd but composite.

Forgetting that 22 is prime

Why it's wrong: Some students think primes must be odd.

Correct: 22 is the only even prime number. It has exactly two factors: 11 and 22.

Interactive Visual

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Practice Problems

15 problems
Problem 1 of 15
Easy
Which of these numbers is prime?

Why It Matters

Prime numbers are the "building blocks" of all whole numbers:
  • Cryptography: Internet security and passwords rely on very large prime numbers
  • Computer Science: Prime numbers help create efficient data structures
  • Nature: Some cicadas emerge every 13 or 17 years (both prime!), probably to avoid predators
  • Mathematics: Every whole number greater than 11 is either prime or a product of primes
Understanding primes helps you simplify fractions, find common denominators, and solve many other math problems!

Real World Applications

Internet Security

When you shop online or log into your bank, your data is protected by encryption that uses very large prime numbers.

Example:

Security systems multiply two huge primes (hundreds of digits each). It's easy to multiply them but nearly impossible to figure out which primes were used.

1Try It Yourself

A simple encryption uses two small primes multiplied together: 5×7=355 \times 7 = 35.

If someone only knows the product is 3535, can you find the two prime factors?

Step 1: Write the mathematical expression

Find two primes that multiply to 3535:

Cicada Life Cycles

Some cicadas live underground for 13 or 17 years before emerging. Scientists believe they evolved prime-numbered life cycles to avoid predators.

Example:

A predator with a 2-year cycle would meet 13-year cicadas only every 26 years. A 3-year predator would meet them only every 39 years!

2Try It Yourself

A cicada has a 17-year cycle. A predator has a 4-year cycle.

How often would the predator and cicada emerge in the same year?

Step 1: Write the mathematical expression

Find the LCM of 1717 and 44:

Key Takeaways

  • 1A prime number has exactly two factors: 11 and itself (examples: 2,3,5,7,112, 3, 5, 7, 11)
  • 2A composite number has more than two factors (examples: 4,6,8,9,104, 6, 8, 9, 10)
  • 3The number 11 is neither prime nor composite
  • 422 is the only even prime number
  • 5To test if a number is prime, check divisibility by primes up to its square root

Frequently Asked Questions

A prime number must have exactly two different factors. The number 11 only has one factor (itself), so it doesn't meet the definition. Also, if 11 were prime, every number would have infinitely many prime factorizations (e.g., 6=2×3=1×2×3=1×1×2×36 = 2 \times 3 = 1 \times 2 \times 3 = 1 \times 1 \times 2 \times 3...).
A prime number must have exactly two different factors. The number 11 only has one factor (itself), so it doesn't meet the definition. Also, if 11 were prime, every number would have infinitely many prime factorizations (e.g., 6=2×3=1×2×3=1×1×2×36 = 2 \times 3 = 1 \times 2 \times 3 = 1 \times 1 \times 2 \times 3...).
No! There are infinitely many prime numbers. The ancient Greek mathematician Euclid proved this over 2000 years ago. The record prime found in 2024 has more than 41 million digits!
Every even number greater than 22 is divisible by 22, so it has at least three factors (11, 22, and itself). This makes all even numbers except 22 composite.

Glossary

Prime number
A whole number greater than 11 with exactly two factors: 11 and itself
Composite number
A whole number greater than 11 with more than two factors
Factor
A number that divides evenly into another number
Divisible
A number is divisible by another if the division results in a whole number with no remainder
Today's challengeOne problem, three tries

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Prime Factorization

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Greatest Common Factor (GCF)

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Least Common Multiple (LCM)

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