Moseley found that, for a given X-ray line, $\sqrt{\nu} = a(Z - b)$, where $a$ and $b$ are constants for that line. The same line is measured for three elements. Element A ($Z = 11$) gives $\nu = 2.50 \times 10^{17}\ \text{Hz}$. Element B ($Z = 41$) gives $\nu = 4.00 \times 10^{18}\ \text{Hz}$. An unknown element X gives $\nu = 1.00 \times 10^{18}\ \text{Hz}$. What is the atomic number of X?