Which has a greater number of atoms a gram of lead or a gram of uranium?

Look at your periodic table.
The element with the lowest number has the most number of atoms making up 1 gram of an element.

The atomic number of Uranium is 92 
The atomic number of Lead is 82

They are both pretty heavy, but Lead has the lower number so you need more atoms of it than you do uranium.

Most atoms = 82 Lead.