What is a compound?

A compound is defined as a substance formed when two or more different types of atoms are chemically bonded together. This means that compounds have a specific chemical composition and properties distinct from the elements that constitute them. For example, water (H₂O) is a compound made of hydrogen and oxygen atoms bonded together in a fixed ratio.

The key aspect of compounds is that the atoms are not just mixed together; they are chemically united, which often changes their individual properties and leads to the formation of new characteristic traits. This chemical interaction can involve covalent or ionic bonds, leading to a stable arrangement of atoms.

In contrast, other choices highlight variations that do not fit the definition of a compound. For instance, substances made of identical types of atoms refer to elements, while an impure substance made from different elements implies a mixture rather than a chemically bonded compound. Lastly, a physical mixture of gases involves no chemical bonding between the participating substances, which further emphasizes why the chosen answer accurately encapsulates the definition of a compound.