`\documentclass{article}
\usepackage{booktabs,siunitx}
\begin{document}
\begin{table} \centering
\caption{Nominal wavelengths (in \SI{}{\nm}) at characteristic emission lines of four different lamps} \label{tab:nominal}
\begin{tabular}{cccc}
\toprule
{Sodium} & {Mercury} & {Cadmium} & {Helium}\\
\midrule
{588.995} & {404.66} & {467.81} & {447.15}\\
{589.590} & {435.83} & {479.99} & {501.57}\\
{} & {546.07} & {508.58} & {587.56}\\
{} & {579.07} & {643.65} & {667.82}\\
{} & {} & {} & {706.52}\\
\bottomrule
\end{tabular}
%\medskip
\caption{Nominal wavelengths (in \SI{}{\nm}) at characteristic emission lines of four different lamps} \label{tab:nominal}
\begin{tabular}{cccc}
\toprule
{Sodium} & {Mercury} & {Cadmium} & {Helium}\\
\midrule
{589} & {406} & {468} & {447}\\
{592} & {436} & {480} & {502}\\
{} & {546} & {509} & {587}\\
{} & {579} & {644} & {668}\\
{} & {} & {} & {707}\\
\bottomrule
\end{tabular}
\end{table}
\end{document}`
\caption deve andare fuori da tabular.
La prossima volta invia un esempio minimo compilabile
Ciao
Ivan