What's the best way to study for chemistry test?

Modern weaponry has increased the number of deaths in wars and violent conflicts.

Phosphorous can form an ion called phosphide, which has the formula p3−. this ion can form an ion called phosphide, which has the formula p3−. this ion properties very similar to those of pforms when a phosphorus atom loses three protonsis called a cationcontains 18 electrons

How does science use models to gain a better understanding of concepts?