# ! Misplaced \noalign. \midrule ->\noalign

This is the table

\documentclass{article}
\usepackage{array}
\newcolumntype{C}{>{$}c<{$}}
\usepackage[skip=0.333\baselineskip]{caption}
\usepackage{booktabs,
amsmath,
siunitx}
\usepackage{txfonts}
\begin{document}

\begin{table}
\caption{Estimated mass of the three Itokawa particles with taking into account their porosity}
\label{Tab3}
\centering
\begin{tabular}{@{} l CCCCC @{}}
\toprule
Sample      &   Silicate density (g/ (\mu m^3))  &  Volume (\mu m^3)    & Estimated mass (g)    & Porosity (%) &    Estimated mass with considering the porosity (g) \\
\midrule
RA-QD02-0014      &  2.196 \times 10^(-12) &  125921  &2.7652 \times10^(-7)    &  0.7  &               2.7458 \times 10^(-7)  \\
RA-QD02-0023       &  2.196  \times 10^(-12)   &    781081   &     1.7153 \times10^(-6)    &    0.0    &       1.7153 \times 10^(-6)     \\
RA-QD02-0047       &    2.196  \times 10^(-12)  &  148570  &   3.2626\times 10^(-7)    &    0.0    &        3.2626 \times 10^(-7)  \\
\bottomrule
\end{tabular}
\end{table}
\end{document}

and the error

! Misplaced \noalign.
\midrule ->\noalign
{\ifnum 0=`}\fi \@aboverulesep =\aboverulesep \global \@...
l.283               \midrule

?
• Regarding @epR8GaYuh comment, have a look at tex.meta.stackexchange.com/questions/228: Your code can not be executed as it is at the moment. People need to add surrounding code. This is considered bad practice here. – Dr. Manuel Kuehner Feb 11 at 12:41
• You have Porosity (%). It probably should be Porosity (\%). – Phelype Oleinik Feb 11 at 12:50
• @Yasamin Use the siunitx package for this sort of thing. – Will Robertson Feb 11 at 13:02
• @Yasamin: \num{2.196d-12} will improve the printing of numbers, use siunitx for this, as suggested by Will Robertson already – Christian Hupfer Feb 11 at 14:10

In addition to fixing the obvious error -- remember that % in TeX and LaTeX is a comment character -- you should

• do a better job of typesetting small numbers using exponential notation -- I suggest you use the \num macro of the siunitx package

• do a better job of typesetting scientific units -- I suggest you employ the \si macro of the siunitx package and place the units on a line by themselves

• allow a line break in at least one of the cell headers, to make the tabular material fit inside the width of the textblock. I suggest you load the tabularx package, use a tabularx environment instead of a tabular environment, and apply a centered version of the package's X column type to the final data column.

• and, last but not least, don't load the obsolete txfonts package. Instead, load the newtxtext and newtxmath packages.

Also, do use fewer words in the caption and various cell headers.

\documentclass{article}
\usepackage{tabularx}
\newcolumntype{C}{>{\centering\arraybackslash}X}
\usepackage[skip=0.333\baselineskip]{caption}
\usepackage{booktabs,siunitx}
\usepackage{newtxtext,newtxmath} % 'txfonts' is obsolete

\begin{document}

\begin{table}
\caption{Estimated mass of Itokawa particles taking
into account their porosity}
\label{Tab3}
\sisetup{round-mode=places,round-precision=3} % apply automatic rounding
\setlength\tabcolsep{3pt} % default: 6pt
\begin{tabularx}{\textwidth}{@{} l cccc C @{}}
\toprule
Sample & Silicate density
& Volume
& Estimated mass
& Porosity
& Estimated mass given porosity \\
& (\si{g\per\micro\meter\cubed})
& (\si{\micro\meter\cubed})
& (g)
& (\%) %% not (%)
& (g)\\
\midrule
RA-QD02-0014 & \num{2.196e-12} & 125921 & \num{2.7652e-7} & 0.7 & \num{2.7458 e-7} \\
RA-QD02-0023 & \num{2.196e-12} & 781081 & \num{1.7153e-6} & 0.0 & \num{1.7153 e-6} \\
RA-QD02-0047 & \num{2.196e-12} & 148570 & \num{3.2626e-7} & 0.0 & \num{3.2626 e-7} \\
\bottomrule
\end{tabularx}
\end{table}
\end{document}

You should use siunitx facilities. I suggest a legend for the table columns, instead of those long headers.

\documentclass{article}
\usepackage[skip=0.333\baselineskip]{caption}
\usepackage{booktabs,amsmath,siunitx}
\usepackage{newtxtext,newtxmath}% better than txfonts

\sisetup{per-mode=symbol}

\begin{document}

\begin{table}[htp]

\caption{Estimated mass of the three Itokawa particles with taking into account their porosity}
\label{Tab3}
\centering
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\textwidth}{
@{\extracolsep{\fill}}
l
S[table-format=1.3e-2]
S[table-format=6.0]
S[table-format=1.4e-1]
S[table-format=1.1]
S[table-format=1.4e-1]
@{}
}
\toprule
Sample  &
{SD (\si{\gram\per\micro\meter\cubed})} &
{V (\si{\micro\meter\cubed})} &
{EM (\si{\gram})} &
{P (\%)} &
{EMP (\si{\gram})} \\
\midrule
RA-QD02-0014 & 2.196e-12 & 125921 & 2.7652e-7 & 0.7 & 2.7458e-7 \\
RA-QD02-0023 & 2.196e-12 & 781081 & 1.7153e-6 & 0.0 & 1.7153e-6 \\
RA-QD02-0047 & 2.196e-12 & 148570 & 3.2626e-7 & 0.0 & 3.2626e-7 \\
\bottomrule