What is formed when elements join together?

When elements join together, they form a new compound. A compound is a substance that results from the chemical bonding of two or more different elements in fixed proportions. During this process, the elements undergo chemical reactions that result in the formation of new properties that are different from those of the individual elements.

For example, when hydrogen and oxygen combine chemically, they form water (H₂O), a compound with unique characteristics that do not resemble those of either hydrogen or oxygen. This transformation involves the formation of chemical bonds—such as covalent bonds—between the atoms of the different elements, creating a stable structure that defines the compound.

The other options refer to different concepts in chemistry. An isotope involves variations of a single element based on differing numbers of neutrons; a physical mixture consists of two or more substances combined without any chemical bonding, allowing them to retain their individual properties; and a catalyst is a substance that accelerates a chemical reaction without being consumed in the process. None of these describe the formation of a new compound.