Find the missing number?

1
2
5
7

The missing number is 3.

The numbers in the sequence are all prime numbers, except for 2, which is a composite number. The sequence starts with 2 and increases by 2 each time. Therefore, the missing number is 3, which is the next prime number after 2.

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A composite number is a natural number greater than 1 that is not prime.