All prime numbers are odd except for the number two. A prime number is defined as any whole number greater than one that has no positive divisors except for one and itself. Since two can only be divided by the numbers one and two, it is prime.
All other prime numbers are odd, because every even number can be divided by two, meaning the number must have at least three possible divisors, making it a composite number. There is no simple formula for determining whether a very large number is prime or composite, but no prime number can end in an even number or zero. With the exception of the number five, no prime number can end in five.