Why is time plotted on the x axis and distance on the y axis?

Because time is independent of distance, and distance is dependent of time.

Explanation:

Usually, on any graph, the independent variable is plotted on the x-axis and the dependent variable is plotted on the y-axis. Because of this, time, which is independent (time happens regardless of any other factor), is on the x-axis while distance, which is the dependent variable (can only take place in time), is on the y-axis.

sodium (na), for example, has one electron in its valence shell. this is an unstable state because that valence shell needs eight electrons to be full. in order to fill its valence shell, find a way to add seven electrons to its valence shell