Skeletal numbers of elements have been introduced as derivatives of the 4n series method. They are based on the number of valence electrons present in the skeletal element. They are extremely useful in deducing possible shapes of skeletal elements in molecules or clusters especially the small to medium ones. For large skeletal clusters, the skeletal numbers may simply be regarded as identity numbers. In carbonyl clusters, they can be used as a guide to facilitate the distribution of the ligands such as CO, H and charges onto the skeletal atoms. A naked skeletal cluster may be viewed as a reservoir for skeletal linkages which get utilized when ligands or electrons get bound to it. The sum of linkages used up by the ligands bound to a skeletal fragment and the remaining cluster skeletal numbers is equal to the number of the skeletal linkages present in the original "naked parent" skeletal cluster. The skeletal numbers can be used as a quick way of testing whether or not a skeletal atom obeys the 8-or 18-electron rules.

Metalloboranes have been categorized by using 4n series method. More than one hundred metalloboranes have been analyzed and characterized by the method. The clusters have been found to be centered within the series range S = 4n+6 to 4n-8. According to the classification, the clusters of series S= 4n+0 are mono-capped, S = 4n-2, bi-capped, S = 4n-4, tri-capped, S = 4n-6, tetra-capped and S= 4n-8, penta-capped. Whereas the known stable boranes have almost non-existent capped boranes, such clusters are prevalent within metalloborane complexes. This implies that metal fragments have the potency of stabilizing the fragile capped boranes.

Golden clusters have been analyzed and categorized using the series method. They have been found to be intensely capped. The complexity of capping is likely responsible for the unique shapes of clusters. The predicted structures of the golden clusters with one or two nuclear indices have been found to be in agreement with the observations from structural determinations. According to the series, the golden clusters have tendency of occurring in an overlapping area which is normally dominated by naked metal clusters. The series approach method is easy and very reliable in analyzing and characterizing clusters. A brief background to the series method was included to ease readability of the paper.

The evolution of the use of the 14/4n series method has resulted in the discovery of skeletal numbers. The skeletal numbers have rendered the testing of the 18-and 8-electron rules much easier and faster. In the same way, they have been very useful in simplifying the categorization of clusters, the prediction of shapes, and the matching of isolobal fragments. Furthermore, the skeletal numbers have made it possible to tentatively assign ligands to individual skeletal elements in such way that the 18-or 8-electron rule is obeyed. Thus, skeletal numbers can be utilized as a simple guide in analyzing and understanding clusters.

Zintl ion clusters have the industrial potential for possible applications. A good understanding of such clusters in terms of their bonding, structures and chemical reactivity is extremely important. This paper attempts to categorize and predict the shapes of Zintl clusters of simple to medium nuclearity using the 4n series method.