CBSE Class 12-science Answered

The empty spaces between the atoms are interstitial sites. Since each is touched by 4 circles, the interstitial sites are also 4-coordinate, and the geometry is also square.  In this example there are the same number of interstitial sites as circles.

Also because the atoms attract one another, there is a tendency to squeeze out as much empty space as possible. The packing efficiency is the fraction of the crystal (or unit cell) actually occupied by the atoms. It must always be less than 100% because it is impossible to pack spheres (atoms are usually spherical) without having some empty space between them.
 

Regarding our square lattice of circles, we can calculate the packing efficiency (PE) for this particular lattice as follows:

The interstitial sites must occupy 100% - 78.54% = 21.46%.

 the arrangement about any single atom in the simple cubic lattice; note that each atom has 6 neighbors, so the atomic coordination number  is 6. The coordination geometry is octahedral (an octahedron has 6 corners). 
  
 
  Actually, the corner atoms touch the one in the center of the face. The name "close packed" refers to the packing efficiency of 74.05%. No other packing can exceed this efficiency (although there are others with the same packing efficiency).

If we stack the cells into a lattice we notice that the atoms form diagonal layers - the reason for the colors is to make these stand out.  Note that diagonal layers also form along our line of sight.  

Actually, the corner atoms touch the one in the center of the face. The name "close packed" refers to the packing efficiency of 74.05%. No other packing can exceed this efficiency (although there are others with the same packing efficiency).

If we stack the cells into a lattice we notice that the atoms form diagonal layers - the reason for the colors is to make these stand out.  Note that diagonal layers also form along our line of sight. Since these cut across the other layers, each layer will contain all three colors.