What are the unique characteristics of carbon, which give it the ability to form a vast array of chemical substances?

Carbon has an exceptional ability to bind with a wide variety of other elements. Carbon makes four electrons available to form covalent chemical bonds, allowing carbon atoms to form multiple stable bonds with other small atoms, including hydrogen, oxygen, and nitrogen.

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answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.