Which one of the following events occurs first in a nerve impulse?
a. potassium ions move to the outside of a neuron.
b. sodium ions move to the inside of a neuron.
c. potassium ions move to the inside of a neuron.
d. sodium ions move to the outside of a neuron.

The answer is b.

Normally, neuronal cell membranes have negative voltages across them (meaning that the inside is more negative relative to the exterior). In the event of an action potential (when a neuron fires), the first thing that happens is that the membrane depolarizes, meaning that the charge of the inside relative to the outside begins to reverse. This happens when voltage-gated ion channels allow sodium ions (Na+, positively charged) to rush through the membrane and into the cell. Thus, the polarity of the membrane charge begins to reverse (inside more positive, outside less positive), which is what we call depolarization.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.