The resting membrane potential of a mammalian axon is around -65mV. Such a potential exists because various ion channels and pumps on the membrane create and maintain a separation of charges between the inside and the outside of the axon. Understanding this separation of charges or gradient for each ion is important for analyzing any ion movement.
Sodium ions have
(very few passive channels).
There are more sodium ions on the outside than the inside, so if given opportunity, i.e. opened gates, sodium ions will flush into the cell, creating an inward current and a
(less negative) from the resting membrane potential.
Potassium ions have
(a lot of passive channels) and there are more potassium ions on the inside than the outside.
Potassium ions would flush out of the cell more when allowed, creating an outward current and a
(more negative) from the resting membrane potential.