Numbers and Number Sense: Part 4: Prime and Composite Numbers

A factor, or divisor, divides a number without leaving a remainder. Positive integers that have exactly two factors are called prime numbers. Another way of saying this is - a prime number is an integer greater than 1 that is divisible only by 1 and the number itself. The number 5 is a prime number. Its only factors are 1 and 5. Two other primes are 23 and 47. The only even number that is prime is 2. Can you think why none of the other even numbers are prime?

Positive integers that have more than two distinct factors are called composite numbers. One example is 4, which has factors of 1, 2, and 4. Another example is 27, which has factors of 1, 3, 9, and 27.

The positive integer 1 is neither prime nor composite since it only has one factor - the number 1 itself.

Which of the following numbers are prime: 5, 8, 17, 19, 21?

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Key Notes:

• Factor or Divisor: divides number with out a remainder
• Prime Numbers: positive integer that have exactly 2 factors - divisible by 1 and itself
• Composite numbers: have more than 2 factors
• Number 1 is not prime nor composite

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