Different arrangements of the circles can lead to different packing efficiencies. Arrange 9 pennies or circles such that the unit cell is now a hexagon around the center coin. This is called the close packing arrangement.
(1) What is the coordination number around the central circle? I say 8
(2) What is the packing efficiency? Consider a close-packed arrangement of circles. The actural unit cell is a parallelogram. The area of a parallelogram can be found by the formula A=bxh. The dimensions of b are easy b=2r. However, h is a little harder although you should be able to figure it out using simple geometry (hint) h=the square root of 3 times r