With use of the tabularray
package with option row{every[4]{5}{-1}}
for inserting small vertical space after each four rows in table body (as supplement to nice @Mico answer (+1), for exercise):
\documentclass{article} % or some other suitable document class
\usepackage{iftex} \ifpdftex\usepackage[T1]{fontenc}\fi
\usepackage{tabularray}
\UseTblrLibrary{booktabs}
\usepackage{upgreek,mhchem}
\hyphenation{iso-prene di-terpene tri-terpene mono-terpene
sesqui-terpene}
\begin{document}
\begin{table}[ht]
\caption{List of compounds in mixed stock solution. }
\smallskip
\textbf{Legend:}
iso: isoprene, mt: monoterpene, omt: oxygenated monoterpene, sqt: sesquiterpene, osqt: oxygenated sesquiterpene, dt: diterpene, tt: triterpene.
\smallskip
\begin{tblr}{width=0.48\textwidth,
baseline = t,
colspec = {@{} Xll @{}},
rowsep = 1pt,
row{every[4]{5}{-1}} = {belowsep=1.5ex},
}
\toprule
Compound & Type & Formula \\
\midrule
Isoprene & iso & \ce{C5H8} \\
$\upalpha$-Pinene & mt & \ce{C10H16} \\
Camphene & mt & \ce{C10H16} \\
Sabinene & mt & \ce{C10H16} \\
$\upbeta$-Pinene & mt & \ce{C10H16} \\
$\upbeta$-Myrcene & mt & \ce{C10H16} \\
$\upalpha$-Phellandrene & mt & \ce{C10H16} \\
3-Carene & mt & \ce{C10H16} \\
$\upalpha$-Terpinene & mt & \ce{C10H16} \\
\textit{m}-Cymene & mt & \ce{C10H14} \\
\textit{p}-Cymene & mt & \ce{C10H14} \\
Ocimene & mt & \ce{C10H16} \\
Eucalyptol & omt & \ce{C10H18O} \\
\textit{D}-Limonene & mt & \ce{C10H16} \\
\textit{o}-Cymene & mt & \ce{C10H14} \\
$\upgamma$-Terpinene & mt & \ce{C10H16} \\
Sabinene Hydrate & omt & \ce{C10H18O} \\
Terpinolene & mt & \ce{C10H16} \\
Linalool & omt & \ce{C10H18O} \\
Fenchol & omt & \ce{C10H18O} \\
Isopulegol & omt & \ce{C10H18O} \\
\\ % blank line
\bottomrule
\end{tblr}\hspace{\fill} % maximize horizontal separation
\begin{tblr}{width=0.48\textwidth,
baseline = t,
colspec = {@{} Xll @{}},
rowsep = 1pt,
row{every[4]{5}{-1}} = {belowsep=1.5ex},
}
\toprule
Compound & Type & Formula \\
\midrule
Isoborneol & omt & \ce{C10H18O} \\
($-$)-Borneol & omt & \ce{C10H18O} \\
($+$)-Borneol & omt & \ce{C10H18O} \\
Menthol & omt & \ce{C10H20O} \\
Terpinen-4-ol & omt & \ce{C10H18O} \\
$\upalpha$-Terpineol & omt & \ce{C10H18O} \\
Citronellol & omt & \ce{C10H20O} \\
Carvacrol & omt & \ce{C10H14O} \\
Geraniol & omt & \ce{C10H18O} \\
Thymol & omt & \ce{C10H14O} \\
($-$)-$\upalpha$-Cedrene & sqt & \ce{C15H24} \\
Caryophyllene & sqt & \ce{C15H24} \\
trans-$\upbeta$-Farnesene & sqt & \ce{C15H24} \\
Humulene & sqt & \ce{C15H24} \\
Valencene & sqt & \ce{C15H24} \\
Nerolidol & osqt & \ce{C15H26O} \\
Guaiol & osqt & \ce{C15H26O} \\
Cedrol & osqt & \ce{C15H26O} \\
($-$)-$\upalpha$-Bisabolol & osqt & \ce{C15H26O} \\
Farnesol & osqt & \ce{C15H26O} \\
Phytane & dt & \ce{C20H42} \\
Squalene & tt & \ce{C30H50} \\
\bottomrule
\end{tblr}
\end{table}
\end{document}
