A process gas (such as a hydrocarbon gas) is flowed through a thermal cracking apparatus to crack the process gas into constituent components (such as hydrogen gas and solid carbon nano-particles, e.g., carbon nano-onions, necked carbon nano-onions, carbon nanospheres, graphene, graphite, highly ordered pyrolytic graphite, single walled nanotubes, and/or multi-walled nanotubes). The thermal cracking apparatus has an elongated heating element disposed within an inner volume along a longitudinal axis thereof. The elongated heating element heats the process gas as it flows within a longitudinal elongated reaction zone to thermally crack molecules of the process gas into the constituent components of the molecules.