Why is the resting membrane potential of a neurone negative when there are positive ions inside the cell?

The resting membrane potential inside the neurone (-70mV) is negative relative the outside! This means that even though there are both Na⁺ and K⁺ ions inside the cell, there are fewer of them (-70mV fewer) inside the cell compared to the outside. There are also more negative ions, e.g. Cl^-, inside the cell compared to outside. The 'membrane potential' (charge) inside the neurone is less than that outside of the neurone, and therefore negative. In summary, the resting membrane potential is negative due to the amounts (concentration) of positive and negative ions inside/outside of the cell, not the charge of the ions themselves.