How could there be an element lighter than hydrogen? Elements are atoms -- nuclei of protons and neutrons orbited by electrons. A hydrogen nucleus is... one proton. Period. Since there are no free quarks...
There wouldn't need to be. Theoretically, quark matter (which is very bizzare and highly unstable) would be intrinsically lighter than hydrogen since particles composed of quark matter would lack protons or neutrons and would instead consist of electrons bound to positively-charged quarks.
It would have some INCREDIBLY bizzare properties, which would depend on its condition, structure and stability. On the scifi side, Mass Effect's "Element Zero" is basically this, a substance that has the odd property of waring space when subjected to an electrical charge; on the alarmist side, there are some theories going around that quark matter is "metastable" such that ordinary matter that comes into contact with quark matter will immediately be converted into quark matter, and then immediately disintegrate (in some cases, deflagrate
) since quark matter cannot sustain alot of the more complex chemical structures found in ordinary matter.