A molecule is a group of two or more atoms that are chemically bonded together. These atoms can be of the same element, like in a oxygen molecule (O₂), or different elements, such as in a water molecule (H₂O). Molecules are the smallest units of a chemical compound that retain its properties. Molecules can vary in size and complexity, ranging from simple diatomic molecules to large biological molecules like proteins and DNA. They play a crucial role in chemistry and biology, as they are the building blocks of matter and are involved in various chemical reactions.