Which element has the greatest density at stp? calcium, chlorine, carbon or copper?

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.