The coordination number of the topmost layer atoms is 7


Each surface atom has two nearest neighbours in the 1st layer, and another four in the layer immediately below, and one directly below it in the third layer ; this gives a total of 7.

To confirm this consider those that have been removed from the layers above - clearly there would have been 4 nearest neighbours in the layer immediately above the surface layer ( equivalent to the four in the layer immediately below ). In addition, there would have been one nearest neighbour directly above each surface atom ( equivalent to the one directly below in the third layer ). Hence, 7 (present) + 5 (removed) = 12 - which is correct !

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