To express \( 20^{1/2} \) in simplest radical form, we first recognize that \( 20^{1/2} \) is equivalent to the square root of 20:
\[ 20^{1/2} = \sqrt{20} \]
Next, we simplify \( \sqrt{20} \) by factoring 20 into its prime factors. The number 20 can be factored as follows:
\[ 20 = 4 \times 5 = 2^2 \times 5 \]
Now we can express the square root of 20 using its factors:
\[ \sqrt{20} = \sqrt{4 \times 5} \]
We can use the property of square roots that states \( \sqrt{a \times b} = \sqrt{a} \times \sqrt{b} \):
\[ \sqrt{20} = \sqrt{4} \times \sqrt{5} \]
Since \( \sqrt{4} = 2 \), we have:
\[ \sqrt{20} = 2 \times \sqrt{5} \]
Thus, the simplest radical form of \( 20^{1/2} \) is:
\[ \boxed{2\sqrt{5}} \]