Coordination numbers in a two dimensional  square close packed structure and hexagonal close packed structure respectively, are

Close packing in two-dimensions can be

Type I :  AAA …… type

The second row may be placed in contact with the first one such that the spheres of the second row are exactly above those of the first row. the spheres of the two rows are aligned horizontally as well as vertically. in this arrangement, each sphere is in contact with four of its neighbours. 

Thus, its coordination number = 4 and is called square close packing in two-dimensions. 

Atom $A(•)$ is in contact with four others. 

Type II :   ABAB …… type

The second row may be placed above the first in a staggered manner such that its spheres fit in the depressions of the first row. 

First row arrangement: A type 

Second row arrangement: B type 

Third row arrangement: A type and so on 

There is less free space and packing is more efficient than the square close packing. Each sphere is in contact wit six of its neighbours and coordination number = 6 in two dimensions. It is called two-dimensional hexagonal close-packing. Voids formed in this case are triangular in shape. If the apex of the triangles downward in the first row then It is upward in the second row.