The number 521 is a prime number because it is not possible to factorize it. In other words, 521 is only divided by 1 or by itself.
source:
http://coolconversion.com/math/prime-numbers/Is_521_a-prime-number?
List of prime numbers before 521 (including 521 itself):
1 2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32 33 34 35 36 37 38 39 40 41 42
43 44 45 46 47 48 49 50 51 52 53 54 55 56
57 58 59 60 61 62 63 64 65 66 67 68 69 70