A2B→2A++B−2,B−2 has complete octet in its dianionic form, thus in its atomic state it has 6 electrons in its valence shell. As it has negative charge, it has acquired two electrons to complete its octet. If the lowest oxidation state of an atom is −2, this typically implies that the atom can gain two electrons to reach a stable configuration. Therefore, the number of valence electrons in that atom would be six, because gaining two electrons would fill its outer shell.