booktabs rules donot show up

revtex has its own rule macros. Do not load the booktabs package and use:

CODE: SELECT ALL  •  OPEN IN ONLINE LATEX EDITOR

\begin{tabular}{ccc}\toprule
    
 \multicolumn{3}{r}{$k_{20}+k_{21}$($\times$10$^{-9}$cm$^{3}$s$^{-1}$)} \\\cline{2-3}
    
  Neutral & Default & New \\\colrule
    
  Kr &4.02 $\pm$ 0.46~ & 4.17 $\pm$ 0.49 \\
    
  Xe & 6.11 $\pm$ 0.72& 6.39 $\pm$ 0.76\\\botrule
    
\end{tabular}