\documentclass[12pt]{article}

\usepackage{booktabs}

\usepackage{siunitx}

\usepackage{listings}

\usepackage{subcaption}

\begin{document}
	\begin{table}[h]
	\centering
	\renewcommand{\arraystretch}{1.25}%
	\begin{subtable}[t]{0.4\textwidth}
		\begin{tabular}{cc}
			\toprule
			Numero d'onda ($\unit{cm^{-1}}$) & Classificazione \\ \midrule
			3204 & weak \\
			3183 & weak \\
			3167 & weak \\ \midrule
			3139 & weak \\ \midrule
			3087 & weak \\
			3072 & weak \\ \midrule
			1856 & strong \\ \midrule
			1511 & weak \\
			1493 & weak \\ \midrule
			1415 & strong \\ \midrule
			1257 & strong \\ \midrule
			1128 & weak \\ \midrule
			938 & weak \\ \midrule
			525 & medium \\ \midrule
			378 & weak \\
			\bottomrule
		\end{tabular}
	\end{subtable}
		\quad
		\begin{tabular}{cc}
			\toprule
			Numero d'onda ($\unit{cm^{-1}}$) & Classificazione \\ \midrule
			3204 & medium \\
			3183 & medium \\
			3167 & medium \\
			3139 & medium \\ \midrule
			3097 & strong \\
			3072 & strong \\ \midrule
			1856 & weak \\ \midrule
			1511 & medium \\ 
			1510 & medium \\
			1493 & weak \\ \midrule
			1411 & weak \\
			1089 & weak \\ \midrule
			938 & weak \\ \midrule
			891 & weak \\ \midrule
			799 & medium \\
			\bottomrule
		\end{tabular}
	\end{table}
\end{document}