A group of atoms within a molecule that interact in predictable ways are called a functional group.

Here's why:

* Functional groups are specific arrangements of atoms within a molecule that are responsible for its characteristic chemical reactions.

* They are like "building blocks" that give the molecule its unique properties.

* For example, the hydroxyl group (-OH) is a functional group that makes molecules soluble in water. The carbonyl group (C=O) is another functional group that is found in ketones and aldehydes.

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