Density = mass/volume
Since at STP, one mole of any gas occupies 22.4 liters, therefore, the volume is equal.
So, we will arrange based on mass.
From the periodic table:
mass of carbon = 12 grams
mass of calcium = 40 grams
mass of chlorine = 35.5 grams
mass of copper = 63.5 grams
Based on this:
copper has the highest density.
If we are talking about moles then the answer to that is 0.22
