\documentclass{article}

\usepackage{booktabs}

\begin{document}

\begin{table}
  \centering
  \begin{tabular}{ l r r}
    \toprule
    \bfseries Title & \multicolumn{1}{c}{$A$} & \multicolumn{1}{c}{$B$} \\
    \midrule
    Total number of atoms & 426 & 520 \\
    Number of electrons   & 260 & 118 \\
    \bottomrule
  \end{tabular}
  \caption{test}
\end{table}

\end{document}