How to calculate the rate of reaction of Sodium and Chlorine
Rate laws are determined via experimental data. Reactant concentrations are held constant while others are modified in order to determine the effect of each reactant's concentration on a reaction. For this reason, one cannot determine the rate law from just a stoichiometric equation.
$$\ce{2Na + Cl2 -&gt; 2NaCl}$$

We can propose reaction mechanisms to make an educated guess of the rate law:
\begin{align}
\ce{Na + Cl2 &amp;-&gt; NaCl + Cl}&amp;&amp;\text{(slow step)}\\
\ce{Na + Cl &amp;-&gt; NaCl}&amp;&amp;\text{(fast step)}\\
\end{align}

Using Hess' law, we can combine these to make sure it equals the original equation.
$$\require{cancel} 
\begin{align}
\ce{  &amp;&amp; Na + Cl2 &amp;-&gt; NaCl + \cancel{Cl}}\\
\ce{+ &amp;&amp; Na + \cancel{Cl} &amp;-&gt; NaCl} \\\hline
\ce{  &amp;&amp; 2Na + Cl2 &amp;-&gt; 2NaCl} \\
\end{align}$$

The coefficients of the slowest elementary step in reaction mechanism determine the exponents of the rate law. Hence:
$$\text{Rate}=k[\ce{Na}]^{1}[\ce{Cl2}]^{1}$$

Or just
$$\text{Rate}=k[\ce{Na}][\ce{Cl2}].$$