1. Let's start by defining a molecule. A molecule is a group of two or more atoms that are chemically bonded together. 2. Molecules can be made of the same type of atoms, like oxygen gas (O2), or different types of atoms, like water (H2O). 3. The atoms in a molecule are held together by chemical bonds, which involve the sharing or transfer of electrons. 4. Molecules are the smallest units of a substance that retain the chemical properties of that substance. 5. For example, a water molecule (H2O) consists of two hydrogen atoms and one oxygen atom bonded together. 6. Molecules can be simple, like oxygen (O2), or complex, like proteins and DNA, which are made of many atoms. 7. Understanding molecules helps us learn about the composition and behavior of matter in general science.