Introduction of essay: talk about the three models used in science (mathematical, conceptual and physical), how do they help scientists understand complex ideas?, introduce the formula for density (as a mathematical model) and the particle model (as a conceptual model ... i think it is...) in science.
Body paragraph of essay: further discuss the properties of the models, link them to the examples (formula of density and the particle model) and compare and contrast (the strengths and weaknesses of each model) e.g. “Mathematical models help scientists to understand the underlying relationship between two concepts. Although a mathematical model is short and concise it is often hard to describe the model conceptually or physically and its therefore harder to get the full picture of how a scientific process works physically. A conceptual model is helpful for intuitive ideas in science like the particle model that would be harder to describe mathematically, due to how broad it is.” Etc etc.
Conclusion paragraph of the essay: “to conclude, models in science are important because they help us to understand difficult and intuitive ideas in a more clear and concise manner.” Etc etc.
Hope this helped - keep in mind that i don’t know the actual difference between conceptual and physical models so look through your books notes for more info for your essay.