\documentclass[10pt,a4paper]{book}
\usepackage{fancyhdr}
\pagestyle{fancy}

\renewcommand{\sectionmark}[1]{
 \markright{\thesection. #1}}
\renewcommand{\chaptermark}[1]{
  \markboth{\chaptername
  \thechapter. \MakeUppercase{#1}}{}}

\fancyhead{}
\fancyhead[LE]{\textsl{\rightmark}}
\fancyhead[RO]{\textbf{\leftmark}}
\fancyfoot{}

\fancyfoot[RE,LO]{Titre du livre}
\fancyfoot[LE,RO]{\thepage}
\renewcommand {\footrulewidth}{0.4pt}

\fancypagestyle{plain} {
\fancyhead{}  % vide l'en-tete
\fancyfoot{}  % vide le pied de page
\renewcommand {\headrulewidth}{0pt}
\renewcommand {\footrulewidth}{0pt}
}
\usepackage[french]{babel}
\begin{document}
\chapter{Chimie}
\section{Ions}

bla bla\newpage
bla bla\newpage
bla bla
\end{document}
\end{code}\end{minipage}\hspace{4.2em}
\begin{minipage}{0.45\linewidth}