Which gas is found in the greatest abundance in the modern atmosphere?

Nitrogen is the gas is found in the greatest abundance in the modern atmosphere. 

Nitrogen is a Group 15 element. Nitrogen makes up about 78% of the atmosphere by volume but the atmosphere of Mars contains less than 3% nitrogen.

What are the options? physical property can be color, smell, opacity, viscosity, density, infra red spectrum, melting and boiling points, attraction and repulson to magnets, etc.

Not a  mixture of two common elements that can be separated by physical means.