To build a model of PCl3, you need two colors of ball. One color will represent each atom. Three balls will be the of one color and one ball will be of a different color.
A model is a visual representation of something that exists in reality. The purpose of a model is to aid a person to conceptualize a given fact.
A molecular model of PCl3 is a visual representation of the molecule. If we have two colors of ball, three of one color and one of a different color, we can have a model of PCl3.
In this model, the three balls of the same color represents chlorine atoms while the lone ball of a different color represents the phosphorus atom.
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