Metal atoms are exported with their valence electrons into MDL mol format.
Consequently, 1 or 2 implicit hydrogens are added to the alkali metals and alkaline earth metals upon export.
By default Tin(Sn) is exported with 2 valence electrons and Thallium (Tl) with one.
Above : Metals exported to MDL mol file using default settings.
Above: Metals in MarvinSketch.