sqrt(51) cannot be represented as a fraction. So it is not rational.
It is not a natural number which is essentially a counting number.
It is not an integer. The set of integers goes from - ∞ < x < + infinity. The numbers are all numbers without any fractions, endless decimals with no predictable sequence.
sqrt(51) is irrational. It has no sequence that you could predict, and it cannot be represented as a fraction.