Why is the mass of a carbon atom greater than a total mass of its protons and electrons?

The definition of mass is the resistance of an object to a change in motion. Protons and electrons have little to no resistance to a change in motion, however a carbon atom does.

I hope this helped!
-Alex

Hope this helps neutrons. this is more of a chemistry question though. neutrons and protons are what make up an atoms mass. the electrons mass is too small to even be taken into account.