How do I center table headings including line breaks?

Question

\documentclass[11pt]{article}  

\usepackage{geometry}                       
\geometry{a4paper}   
\usepackage{mathpazo} 
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{booktabs}
\usepackage{tabularx}

\begin{document}

\begin{table}[htdp]

\centering
\caption{Absorbances of samples of different Concentrations\label{mytable}}
\begin{tabular}{lll} 
\toprule
Sample & Concentration \linebreak (mg/L) & Absorbance at \linebreak $\lambda_{max}$ \\ 
\midrule
blank  & 0.0000   & 0.000 \\                                                                   
a      & 0.2849   & 0.059 \\                                                                   
b      & 0.7122   & 0.160 \\                                                                
c      & 1.424    & 0.311 \\                                                                
d      & 2.137    & 0.468 \\                                                                  
e      & 2.849    & 0.608 \\ 
\bottomrule
\end{tabular}
\end{table}

\end{document}

Why don't my \linebreaks work?

EDIT: I tried to change the code to a two column table

\documentclass[11pt]{article}  
\newcommand{\specialcell}[2][c]{\begin{tabular}[#1]{@{}c@{}}#2\end{tabular}} 
\usepackage{siunitx}
\usepackage{amsmath}
\usepackage{booktabs}

\begin{document}
\begin{table}       
    \centering
    \caption{Absorbances of 20 \si{\ml} sample at different Wavelengths}
    \label{mytable}
    \begin{tabular}{lS[table-format=1.3]} 
        \toprule
        {\specialcell[t]{Wavelength \\in \si{\nano\meter}}}  & Absorbance   \\ 
        \midrule
        400      & 0.240  \\                                                                   
        425      & 0.382  \\                                                                   
        450       & 0.486  \\                                                                
        475      & 0.574  \\                                                                
        500      & 0.608  \\                                                                  
        505      & 0.608 \\
        510      & 0.602 \\ 
        525      & 0.508 \\
        \bottomrule
    \end{tabular}
\end{table} 
\end{document}  

But it throws and error when I add an 'e' to Absorbance

Answer 1

\linebreak is not a command which is meant for tables. You will have to use a p column here or you define a table for each cell. In this answer it has been shown nicely, how to define a command for that.

% arara: pdflatex

\documentclass[11pt,a4paper]{article}     
\usepackage{mathpazo} 
\usepackage{amsmath}
\usepackage{booktabs}
\usepackage{caption}
\newcommand{\specialcell}[2][c]{%
    \begin{tabular}[#1]{@{}c@{}}#2\end{tabular}} % replace c bei l if you want the text to appear ragged right (flush left)
\usepackage{siunitx}

\begin{document}    
    \begin{table}       
        \centering
        \caption{Absorbances of samples of different Concentrations}
        \label{mytable}
        \begin{tabular}{lS[table-format=1.4]S[table-format=1.3]} 
            \toprule
            Sample & {\specialcell[t]{Concentration\\in \si{\milli\gram\per\liter}}} & {\specialcell[t]{Absorbance at\\$\lambda_\text{max}$}} \\ 
            \midrule
            blank  & 0.0000   & 0.000 \\                                                                   
            a      & 0.2849   & 0.059 \\                                                                   
            b      & 0.7122   & 0.160 \\                                                                
            c      & 1.424    & 0.311 \\                                                                
            d      & 2.137    & 0.468 \\                                                                  
            e      & 2.849    & 0.608 \\ 
            \bottomrule
        \end{tabular}
    \end{table} 
\end{document}