By setting them equal to each other.
The number of electrons in a completely filled second shell of an atom is 8.
Hope this helped.
In the most common isotope of fluorine, fluorine-19, it has 19 electrons, 9 protons and 10 neutrons. Hope this helps you! Good luck!!
Do you mean "deciding when the continents shifted"? If so, the fossils of fish and other animals found in mountain belts show when water was surrounding a certain landmass. From here scientists can determine when this land mass separated.