Build a two-dimensional crystal that contains six formula units of sodium chloride in the solid state with just a square

The compound NaCl comprises Na+ and Cl- ions. The unit cell of the crystal comprises sodium and chloride ions. The chloride ions are found at the cube's corners and hold the centers of the faces. The coordination number of the crystal structure is 6.  

The sodium ion is smaller in comparison to the chloride ion size. The composition comprises the six formula units of sodium chloride. Each formula unit of the solid comprises one chloride and one sodium ion. So, six formula unit comprises six sodium and six chloride ions.