You don't need tabularx
, which is the wrong tool for a numeric table. Use tabular*
and siunitx
facilities.
I provide two realizations, the second one without extending the table up to the margins, which seems better.
\documentclass{article}
\usepackage{booktabs,siunitx}
\sisetup{separate-uncertainty}
\begin{document}
\begin{table}[htp]
\caption{Something that describes the table}\label{whatever}
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\linewidth}{
@{\extracolsep{\fill}}
l
S[table-format=6.0,group-four-digits]
S[table-format=2.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=2.2]
S[table-format=1.2(2)]
S[table-format=1.2]
}
\toprule
Probe & {MW} & R & \multicolumn{4}{c}{V (\si{mL/g})} & {K} \\
\cmidrule{4-7}
& {(\si{g/mol})} & {(\si{nm})} & {1} & {2} & {3} & {avg.} & {(--)} \\
\midrule
Label 1 & 400 & 0.00 & 0.00 & 2.34 & 12.00 & 1.32 \pm 0.42 & 1.00 \\
Label 2 & 10000 & 1.50 & 0.99 & 3.00 & 15.00 & 1.32 \pm 0.42 & 1.00 \\
Label 3 & 500000 & 18.45 & 0.99 & 2.00 & 20.00 & 1.32 \pm 0.42 & 1.00 \\
\bottomrule
\end{tabular*}
\end{table}
\begin{table}[htp]
\centering
\caption{Something that describes the table}\label{whatever2}
\begin{tabular}{
@{}
l
S[table-format=6.0,group-four-digits]
S[table-format=2.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=2.2]
S[table-format=1.2(2)]
S[table-format=1.2]
@{}
}
\toprule
Probe & {MW} & R & \multicolumn{4}{c}{V (\si{mL/g})} & {K} \\
\cmidrule{4-7}
& {(\si{g/mol})} & {(\si{nm})} & {1} & {2} & {3} & {avg.} & {(--)} \\
\midrule
Label 1 & 400 & 0.00 & 0.00 & 2.34 & 12.00 & 1.32 \pm 0.42 & 1.00 \\
Label 2 & 10000 & 1.50 & 0.99 & 3.00 & 15.00 & 1.32 \pm 0.42 & 1.00 \\
Label 3 & 500000 & 18.45 & 0.99 & 2.00 & 20.00 & 1.32 \pm 0.42 & 1.00 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}

You can center the header in the first column by using \multicolumn{1}{c}{Probe}
(or \multicolumn{1}{@{}c}{Probe}
for the second realization, where we remove the padding on the left of the first column).
I added also a trick for pulling the units nearer the header.
\documentclass{article}
\usepackage{amsmath,booktabs,siunitx}
\sisetup{separate-uncertainty}
\newcommand{\tabhead}[1]{%
\smash[b]{%
\begin{tabular}[t]{@{}c@{}}#1\end{tabular}%
}%
}
\begin{document}
\begin{table}[htp]
\caption{Something that describes the table}\label{whatever}
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\linewidth}{
@{\extracolsep{\fill}}
l
S[table-format=6.0,group-four-digits]
S[table-format=2.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=2.2]
S[table-format=1.2(2)]
S[table-format=1.2]
}
\toprule
\multicolumn{1}{c}{Probe} &
{\tabhead{MW \\ (\si{g/mol})}} &
{\tabhead{R \\ (\si{nm})}} &
\multicolumn{4}{c}{V (\si{mL/g})} &
{\tabhead{K \\ (--)}} \\
\cmidrule{4-7}
&&& {1} & {2} & {3} & {avg.} \\
\midrule
Label 1 & 400 & 0.00 & 0.00 & 2.34 & 12.00 & 1.32 \pm 0.42 & 1.00 \\
Label 2 & 10000 & 1.50 & 0.99 & 3.00 & 15.00 & 1.32 \pm 0.42 & 1.00 \\
Label 3 & 500000 & 18.45 & 0.99 & 2.00 & 20.00 & 1.32 \pm 0.42 & 1.00 \\
\bottomrule
\end{tabular*}
\end{table}
\end{document}

X
.