What causes convection currents inside earth

Differential heating and pressure.

Convection currents occur when lighter (or not as dense) warm materials rise over cooler, more dense, and heavier material sinks below it. These movements occur inside the mantle, or the third from inner-most layer of the Earth, and are often the cause of tectonic plate moment.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds