Give an example of a physical model, and explain one limitation of the model. then give an example of a mathematical model, and explain one limitation.

It's literally that. A physical model is some situation or object that can be possible touch. Instead of drawing a car that had a new design of some sort, you could build a model of it to illustrate the idea. The limitation of physical model is they look like a real thing. A mathematical model is a model that records data or information. An example of this is a graph. An equation can be also a example of mathematical model like Newton's Model of the Gravitational Energy is E= -mGM/r = mu/r. 

Outermost (valence) electrons

The molar mass of a substance is defined as the mass of 1 mol of that substance, expressed in grams per mole, and is equal to the mass of 6.022 × 10 23 atoms, molecules, or formula units of that substance.