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**Hint:**First draw the structure of bcc, fcc, ccp and hcp and then find the number of its nearest neighbor. That is your coordination number.

Complete step by step answer:

Before knowing the coordination number, we need to understand what the unit cell is. The simplest repeating unit in a crystal is called a unit cell. Each unit cell is defined in terms of lattice points-the point in space about which the particles are free to vibrate in a crystal. There are 14 different types of crystal unit cell structures or lattices that are found in nature. They are called Bravais lattices. However, most metals and many other solids have unit cell structures described as body centered cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp), since these structures are most common in nature.

-Let us first find out what the coordination number is. Coordination number can be defined as the number of atoms or ions immediately surrounding a central atom or ion in a complex or crystal.

-It is found that in covalent bonded molecules and polyatomic ions, the coordination number is determined by just counting the number of bonded atoms.

-Also, there are no clear bonds between pairs of atoms-the metal atom usually packs together as closely as possible and are “glued” together through delocalized electrons.

-bcc stands for body centered cell and the coordination number is 8.

-fcc stands for face centered cell and the coordination number is 12.

-ccp stands for cubic close packed and the coordination number is 12.

-Similarly, hcp stands for hexagonal close packed cell and the coordination number is 12.

Additional Information:

Packing efficiency is defined as the percentage of space occupied by constituent particles packed inside the lattice. High the coordination number, higher is the packing efficiency.

Type of packing | Packing efficiency | No. of atoms per unit cell |

bcc | 68% | 2 |

fcc | 74% | 4 |

ccp | 74% | 6 |

hcp | 74% | 6 |

**Note:**It should be noted that not to confuse coordination number and number of atoms per unit cell. Both are different concepts. For example, the number of atoms per FCC unit cell is 4.

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