矩形节点问题

Question

请问箭头出了什么问题？我该如何修复它？

\documentclass {article}   
\usepackage {tikz}  
\usetikzlibrary {positioning}  
\definecolor {processblue}{cmyk}{0.96,0,0,0}  
\begin {document}  
\begin {center}  
\begin {tikzpicture}[-latex ,auto ,node distance =1cm and 2cm ,on grid ,
semithick ,
state/.style ={ rectangle ,top color =white , bottom color = white ,
draw,black, text=black , minimum width =1 cm}]  
\node(C);  
\node[state] (A) [above left =of C] {$\mathrm{CaO}$};  
\node[state] (B) [above right =of C] {$\mathrm{Ca(OH)_2}$};  
\node[state] (D) [below left =of C] {$\mathrm{Ca}$};  
\node[state] (E) [below right =of C] {$\mathrm{CaCl_2}$};  
\node[state] (F) [above right =of A] {$\mathrm{CaCO_3}$};  
\path (A) edge [state] node[above] {$3$} (B);  
\path (D) edge [state] node[left] {$2$} (A);  
\path (D.5) edge [state] node[above] {$5$} (E.5);  
\path (E.-5) edge [state] node[below] {$1$} (D.-5);  
\path (B) edge [state] node[right] {$4$} (E);  
\path (F) edge [state] node[left] {$7$} (A);  
\path (B) edge [state] node[right] {$6$} (F);  

\end{tikzpicture}  
\end{center} 
\end{document}  

Answer 1

请尝试以下操作：

\documentclass {article}   
\usepackage {tikz}  
\usetikzlibrary {positioning}  
\definecolor {processblue}{cmyk}{0.96,0,0,0}  
\begin {document}  
\begin {center}  
\begin {tikzpicture}[-latex ,auto ,node distance =1cm and 2cm ,on grid ,
semithick ,
state/.style ={ rectangle ,top color =white , bottom color = white ,
draw,black, text=black , minimum width =1 cm}]  
\node(C);  
\node[state] (A) [above left =of C] {$\mathrm{CaO}$};  
\node[state] (B) [above right =of C] {$\mathrm{Ca(OH)_2}$};  
\node[state] (D) [below left =of C] {$\mathrm{Ca}$};  
\node[state] (E) [below right =of C] {$\mathrm{CaCl_2}$};  
\node[state] (F) [above right =of A] {$\mathrm{CaCO_3}$};  
\path (A) edge [state] node[above] {$3$} (B);  
\path (D) edge [state] node[left] {$2$} (A);  
\path (D.5) edge [state] node[above] {$5$} (E.175);  
\path (E.185) edge [state] node[below] {$1$} (D.355);  
\path (B) edge [state] node[right] {$4$} (E);  
\path (F) edge [state] node[left] {$7$} (A);  
\path (B) edge [state] node[right] {$6$} (F);  

\end{tikzpicture}  
\end{center} 
\end{document}