If we compare this coordination number with that obtained for the (100) surface, it is worth noting that the surface atoms on a more open ("rougher") surface have a lower CN this has important implications when it comes to the chemical reactivity of surfaces.ĭo the atoms in the second layer have the bulk coordination ? Hence, 7 (present) + 5 (removed) = 12 - which is correct ! In addition, there would have been one nearest neighbour directly above each surface atom ( equivalent to the one directly below in the third layer ). 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 ). Rationale: 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. The coordination number of the topmost layer atoms is 7 What is the coordination number of the topmost layer atoms on the fcc(110) surface? Answer In this case, the determination of atomic coordination numbers requires a little more careful thought : one way to double-check your answer is to remember that the CN of atoms in the bulk of the fcc structure is 12, and then to subtract those which have been removed from above in forming the surface plane. The preceding diagram illustrates some of those second layer atoms, exposed at the bottom of the troughs. This means that the atoms in the underlying second layer are also, to some extent, exposed at the surface the distance between atoms is equal to twice the metallic(atomic) radius, but in the orthogonal direction there is a substantial gap between the rows. It is clear from this view that the atoms of the topmost layer are much less closely packed than on the (100) surface - in one direction (along the rows) the atoms are in contact i.e. The diagram has been rotated such that the rows of atoms in the first atomic layer now run vertically, rather than horizontally as in the previous diagram. The diagram below shows the conventional birds-eye view of the (110) surface - emphasizing the rectangular symmetry of the surface layer atoms. The (110) surface is obtained by cutting the fcc unit cell in a manner that intersects the x and y axes but not the z-axis - this exposes a surface with an atomic arrangement of 2-fold symmetry. On-top sites (above a single metal atom)ĭepending upon the site occupied, an adsorbate species (with a single point of attachments to the surface) is therefore likely to be bonded to either one, two or four metal atoms.
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The surface offers various adsorption sites for molecules which have different local symmetries and lead to different coordination geometries - specifically there are:.The surface is relatively smooth at the atomic scale.There are several other points worthy of note : This contrasts with the CN of metal atoms in the bulk of the solid which is 12 for a fcc metal.
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Rationale: Each surface atom has four nearest neighbors in the 1st layer, and another four in the layer immediately below a total of 8. The coordination number of the surface layer atoms is 8 What is the coordination number of the surface layer atoms on the fcc(100) surface ? Answer