What is the 3rd coordination number in FCC unit cell? and how?
In an fcc lattice the face centre atoms are the nearest atoms and one corner atom is surrounded by 4 faces in the x-plane, 4 faces in the y-plane and 4 faces in the z-plane. So, every corner atom is surrounded by (4 × 3) = 12 face centre atoms. Since they are the nearest they form the coordination number.