What is a group of two or more atoms held together by a chemical bond?

A group of two or more atoms held together by a chemical bond is known as a molecule. Molecules can consist of the same type of atoms, such as oxygen (O2), or different types of atoms, such as water (H2O), which is made up of hydrogen and oxygen. The defining feature of a molecule is that the atoms within it are bonded together chemically, which distinguishes them from other collections of atoms or substances.

While a compound is also a type of molecule, characterized specifically by containing at least two different types of atoms, it does not encompass the broader definition of molecules, as not all molecules are compounds. An element represents a single type of atom and cannot consist of two or more atoms bonded together. A mixture, on the other hand, involves a physical combination of different substances that are not chemically bonded. Thus, the term "molecule" accurately captures the essence of a group of two or more atoms connected by chemical bonds.