When two or more elements unite chemically, they form what?

When two or more elements unite chemically, they form a compound. A compound is a substance that consists of two or more elements that are chemically bonded together in a fixed ratio, resulting in distinct chemical properties different from the individual elements. This bonding can occur through various types of chemical interactions, such as ionic or covalent bonds.

In contrast, a solution is a homogenous mixture of two or more substances but does not involve any chemical combination between those substances. A mixture refers to a physical combination of different elements or compounds, where each component retains its individual properties and can be separated by physical means. An element, on the other hand, is a pure substance that cannot be broken down into simpler substances by chemical means, and represents the most basic form of matter.

Therefore, the formation of a compound signifies a chemical reaction where the properties of the resulting compound differ fundamentally from those of the individual elements that created it.