In element 173 (unsepttrium), the outermost electron might enter the 6g 7/2, 9p 3/2, or 10s subshells. Because spin–orbit interactions would create a very large energy gap between these and the 8p 3/2 subshell, this outermost electron is expected to be very loosely bound and very easily lost to form a 173 cation. As a result, element 173 is expected to behave chemically like an alkali metal, and one that might be far more reactive than even caesium (francium and element 119 being less reactive than caesium due to relativistic effects): the calculated ionisation energy for element 173 is 3.070 eV, compared to the experimentally known 3.894 eV for caesium. Element 174 (unseptquadium) may add an 8d electron and form a closed-shell 174 cation; its calculated ionisation energy is 3.614 eV.