The energies and structures of the 28-electron tetraatomic molecules, composed of the first row nonmetallic elements : N-4 (1), CN2O (2), BFN2 (3), C2O2 (4), B2F2 (5), CBFO (6), C2FN (7), and BNO2 (8) have been studied uniformly by ab initio methods including coupled-cluster theory. New estimates of the stability of the N-4 isomers, tetraazatetrahedrane and tetraazacyclobutadiene, are presented, and a new tripler NNNN open-chain isomer has been established computationally. Potential energy surfaces of the nonpolar 1 and the polar 2 are compared. Three-membered cyclic C-2v fluorodiazaboririne has been found to be the most stable isomer similar to diazirinone (Chem. Phys. Lett. 1994, 227, 312). Linear triplet CO and BF dimers, OCCO, FBBF, and OCBF, are the most stable forms of 4, 5, and 6, respectively. The singlet cyanofluoromethylene, NCCF, the global energy minimum of 7, is 7 kcal/mol more stable than isomeric CNCF and 10 kcal/mol lower in energy than 3-fluoroazacycloprop-2-ylidene. The singlet and triplet forms of nitrosoboroxide, OBNO, the most stable isomers of 8, were found to have similar energies, within 1 kcal/mol, and the isomeric triplet OBON lies only 4 kcal/mol above OBNO. Singlet-triplet energy separations and dissociation energies to diatomic fragments are compared for the series of linear 14- and 14-electron (NNNN, NNCO, OCCO, FBBF, and OCBF) and open-chain 13- and 15-electron (NCNO, OBCF, NCCF, and OBNO) dimers. Trends in the chemical bonding in the series of 28-electron tetraatomic molecules, 1-8, stability, and possible synthetic routes, are discussed.