The sodium-potassium pump actively transports three ${Na}^{+}$ ions out of the axon for every two $K^{+}$ ions it transports in.

At rest, the membrane is significantly more permeable to $K^{+}$ ions than to ${Na}^{+}$ ions.

The concentration of ${Na}^{+}$ ions is higher inside the axoplasm than in the extracellular fluid when the neuron is at rest.

The resting membrane potential is negative inside the axon relative to the outside, typically around $-70mV$.