Re: What is the lightest solid material?

That is a very interesting question.  The solid containing the least amount 
of mass per number of particles would be solidified elemental Hydrogen.  
Without going into too much detail, Hydrogen can only exist as a solid at 
extremely low temperatures, extremely high pressures, or a complimentary 
combination of both.  It is the first element in the periodic table, a gas 
at room temperature and pressure (Pressure = 1 atmosphere), and it contains 
the fewest number of subatomic particles.  

The lightest solid at room temperature and atmospheric pressure is Lithium, 
Li in the periodic table.  It is the 3rd element in the periodic table and 
has a molecular weight of approximately 6.94 grams/mole. This means that if 
you have 602,000,000,000,000,000,000,000 (6.02X10^23) atoms of Lithium it 
would contain a mass of approximately 6.94 grams.

Note: If you need the weight of a substance, it is necessary to multiply 
the mass of the object by its acceleration.  Acceleration due to gravity is 
9.8 m/s^2(g ~ 9.8 meters/second^2).  
Weight = Force = mass * acceleration. 

Depending on the details behind your question, the answer will vary. 
Solidified Hydrogen is the lightest solid but the temperatures and 
pressures needed to produce this state would not be easy to obtain.  At 
room temperature and atmospheric pressure, the lightest solid material is 
Lithium-a metal.

If you are interested in more information on the periodic table of 
elements, check out this website.
http://www.shef.ac.uk/chemistry/web-elements/index-fr.html