The number 19 is the eighth prime number. A prime number is a positive, whole number that is only divisible by one and itself. Nineteen is also the seventh Mersenne prime exponent, which is a prime number that is one less than a power of two.
Out of the first 100 numbers, 25 of them are prime numbers. The opposite of a prime number is a composite number. Because all even numbers are divisible by two, the number two is the only even prime number. Similarly, no prime numbers that are higher than five end in five. One is a unique number because it is neither a prime number nor a composite number.