2 Na (s) + Cl2 (g) → 2 NaCl (s) is an example of a physical or chemical change?

Chemical change.

Explanation:

Physical Change                                                                                              

The changes that occur only due to change in shape or physical properties but their chemical or internal composition remain unchanged.  

Example :

Water converting to Ice

Water converting to gas

In this water molecule remain the same only they rearrange themselves that change its state of mater not composition

Chemical change:

The changes, that occur due to change in the composition of a substance and result in a different compound is known as chemical change.

These changes are irreversible

These changes occur due to chemical reactions  

These may not be observed with naked eye

Example:

Combustion of fuel or wood: that oil or wood convert into energy, CO2 and ash in case of wood

The reaction of between sodium and chlorine:

2Na(s) + Cl₂(g) > 2NaCl(s)

particle   p is an electron and particle q is a neutron