First-principles electronic structure calculations have been carried out on (formula presented) atoms using ultrasoft pseudopotentials, a plane wave basis and generalized gradient approximation for the exchange-correlation energy, to elucidate the growth behavior and the evolution of the electronic structure. Our results show that clusters with (formula presented) prefer high coordination structures with hexagonal antiprisms except for (formula presented) which has a distorted body centered cubic structure (bcc). Clusters with (formula presented) favor lowest spin configurations. Of particular importance is the finding that icosahedral growth is not favored in these clusters. There are large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps for clusters with 4, 6, 8, 10 and 16 atoms. These are in agreement with the observed low reactivities as well as the photoemission data of 6-, 8-, 10- and 16-atom clusters. Clusters with 4, 8, 10, 15 and 16 atoms are found to be magic, suggesting the importance of the atomically closed shell bcc structure of (formula presented) even though the HOMO-LUMO gap is small. However, (formula presented) is not magic. A dimerization behavior is obtained in (formula presented) and it is found to be even more prominent in the isoelectronic (formula presented) But, this tendency is absent in (formula presented) The calculated binding energies, electronic structures and fragmentation behavior are in good agreement with available experimental data. {\textcopyright} 2002 The American Physical Society.