Alignment of chemical equations

Wouldn't it be easier to use a chemistry package like mhchem (together with the already proposed amsmath) instead of tensor? mhchem's \ce macro is defined especially for use in alignment environments like {align}. For text between aligned equations one can use amsmath's \intertext{}.

\documentclass{book}
\usepackage[version=4]{mhchem}
\usepackage{amsmath}

\begin{document}
\begin{align*}
 \ce{^{14}_7N + n &-> ^{11}_5B  + ^4_2H}
 \intertext{Some text in between that is not aligned or anything.}
 \ce{
   ^{14}_7N + n &-> ^{12}_6C  + ^3_1H \\
   ^{16}_8O + n &-> ^{10}_4Be + ^7_4Be
 }
\end{align*}

\end{document}

Edit: In order to get the first equation numbered but not the second and third you have basically two options:

1. Use {align} (without the star!) and suppress the numbers of the second and third equation via \notag or \nonumber.
2. Use {align*} and add the number for the first manually by saying \tag{\refstepcounter{equation}\theequation}

Personally I prefer the first variant. BTW: if you haven't read it yet you might want to have a look at “Math Mode” by Herbert Voß.

\documentclass{book}
\usepackage[version=4]{mhchem}
\usepackage{amsmath}

\begin{document}

\begin{align}
 \ce{^{14}_7N + n &-> ^{11}_5B  + ^4_2H}
 \intertext{Some text in between that is not aligned or anything.}
 \ce{
   ^{14}_7N + n &-> ^{12}_6C  + ^3_1H \nonumber \\
   ^{16}_8O + n &-> ^{10}_4Be + ^7_4Be \notag
 }
\end{align}

\begin{align*}
 \ce{
   ^{14}_7N + n &-> ^{11}_5B  + ^4_2H \tag{\refstepcounter{equation}\theequation}
}
 \intertext{Some text in between that is not aligned or anything.}
 \ce{
   ^{14}_7N + n &-> ^{12}_6C  + ^3_1H \\
   ^{16}_8O + n &-> ^{10}_4Be + ^7_4Be 
 }
\end{align*}

\end{document}

Try the align* environment, provided by amsmath package. To change the position of the aligment, change the & symbol.

\documentclass{book}
\usepackage{tensor}
\usepackage{amsmath}

\begin{document}
\begin{align*}
   &\nuclide[14][7]{N} + n \to \nuclide[11][5]{B}  + \nuclide[4][2]{H}\\
   &\nuclide[14][7]{N} + n \to \nuclide[12][6]{C}  + \nuclide[3][1]{H}\\
   &\nuclide[16][8]{O} + n \to \nuclide[10][4]{Be} + \nuclide[7][4]{Be}
\end{align*}
\end{document}