Formation of an H⁺ gradient:

All of the electron carriers in the electron transport chain interact according to their redox potentials.  Every time while an electron transfers occurs, the accepting carrier has a higher affinity for electrons than the donating carrier. Therefore there is a net flow of electrons from NADH (most negative redox potential, least affinity for electrons) to oxygen (most positive redox potential, highest affinity for electrons). These ensure a unidirectional flow of electrons.  Moreover, note that each cytochrome, each FeS middle and each copper atom can carry only one electron but each NADH donates two electrons.  Additionally, each molecule of oxygen (O₂) needs to accept four electrons to be reduced to a molecule of water, H₂O. The  several  components   are  arranged  in  such  a  manner  as  to  allow  their different electron-handling properties to work in harmony.

The modification in redox potential along the chain is a measure of the free energy modification occurring. The potential falls for instance becomes more positive during the chain but majorly in three huge steps which correspond to the three major protein complexes:  Complex I, III or IV. The big free energy change at each of these three steps and only these three steps is huge enough to pump H⁺ ions from the mitochondrial matrix across the inner mitochondrial membrane and into the intermembrane space. Therefore, each of these three complexes is an H⁺ pump driven by electron transport (Figs 1 and 2). Whole, thus, electron transport along the chain from NADH releases energy that is used to create an H⁺ gradient.