I want write the following sentences in LaTeX.
n_eandn_iare the density of electrons and ions;m_\alphaandZ_\alphaare the mass and charge number of species\alpha;v_eandv_iare the average (drift) velocity of electron and ion;T_eand T_i are the electron and ion temperatures; Temperature range(1.5<z<=10).
Please advice me
You almost do what you like to achieve. See, if this works for you (I just ad $ before and after your variables, which put them in math environment):
\documentclass{article}
\begin{document}
$n_e$ and $n_i$ are the density of electrons and ions; $m_{\alpha}$ and $Z_{\alpha}$ are the mass and charge number of species $\alpha$; $v_e$ and $v_i$ are the average (drift) velocity of electron and ion; $T_e$ and $T_i$ are the electron and ion temperatures; Temperature range $(1.5<z\le10)$.
\end{document}
Above example gives:
\leqrather than<=perhaps. – Torbjørn T. Jan 28 '16 at 07:08