When two or more atoms join together chemically, they form a molecule. A molecule can consist of the same type of atoms, like oxygen gas (O2), or different types of atoms, like water (H2O), which is made of hydrogen and oxygen atoms.
The process of atoms joining together involves chemical bonds, which can be covalent bonds (where atoms share electrons) or ionic bonds (where electrons are transferred from one atom to another). These bonds are fundamental to creating the compounds that make up all matter in our universe. The way atoms combine determines the properties of the resulting molecule, including its reactivity, state of matter, and other chemical properties.