Fourth Grade · Math · week 7

Prime or composite?

Classify whole numbers as prime or composite by finding all of their factors.

A prime number has exactly two factors: 1 and itself. A composite number has more than two factors. The number 1 is neither prime nor composite, since it only has one factor (itself).

To decide, list all the factors of a number. If the list stops at just 1 and the number itself, it is prime. If anything else divides in evenly, it is composite.

📋 For the teacher
Teach it (10–15 min):
  • Have them list every factor of a number methodically before deciding prime or composite; guessing leads to mistakes.
  • Point out that 1 is neither prime nor composite since "exactly two factors" does not describe it.
  • Highlight 2 as the only even prime so it does not get miscounted as composite out of habit.
  • Keep last week factor pairs handy; classifying prime or composite is just one more question to ask about the same list.
Talk about it:
  • Why is 1 neither prime nor composite?
  • Why is 2 the only even prime number?
  • How can a full list of factor pairs prove a number is composite?

Mastery looks like: They correctly classify numbers as prime or composite by finding their factors and explaining the reasoning.

Common stumbles: Believing 1 is prime; forgetting 2 is prime because it is even; stopping the factor check too soon and mislabeling a composite number as prime.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
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