\begin{table}%t3
%%\centering

\par
\caption{\label{Gas/Surface model - starting abundances}Gas/Surface model -- starting abundances.}
\begin{tabular}{ll} 
\hline \hline \noalign{\smallskip}
Species     & Abundance w.r.t. (H + H$_2$) \\
\hline
H           & $1.00$ \\
He          & $1.40 \times 10^{-1}$ \\
C           & $7.30 \times 10^{-5}$ \\
O           & $1.76 \times 10^{-4}$ \\
N           & $2.14 \times 10^{-5}$ \\
Mg          & $7.00 \times 10^{-9}$ \\
\hline
\end{tabular}
\end{table}