Is it hydrogen atoms or electrons that are passed along an electron transport chain?
As explained on the Wikipedia page of the electron transport chain:


  An electron transport chain (ETC) is a series of complexes that transfer electrons from electron donors to electron acceptors via redox (both reduction and oxidation occurring simultaneously) reactions, and couples this electron transfer with the transfer of protons (H+ ions) across a membrane.


Electron are thus transfered from the electron donor (NADH, H$^+$ produced by the TCA) to the electron acceptor (O$_2$) through the successive enzymatic complexes of the electron transport chain.

Complexes I, III and IV are proton (H$^+$) pumps: they use the energy gained thank to the electron transport in order to translocate protons across the membrane.