Atom is the smallest unit of matter. I mean everything in the universe is made up of atoms. For example, plants, animals, food and water etc. Now here arises an interesting question, if all these things are made of atoms, then why all these things in the universe are different? I mean why not they are the same. Why plants are different from animals? It is because all the atoms are not the same. There are about 118 different kinds of elements having different atoms. Different combination of atoms produces different substances. For example, consider carbon, hydrogen and oxygen. These are the three different types of atoms. When carbon and oxygen combine chemically, they produce carbon dioxide gas. When hydrogen and oxygen combine chemically, they form water (H2O). Thus, different combination of atoms produces different substances.

When an atom or group of atoms exist freely, it forms a molecule. A molecule is usually considered a group of atoms stuck together usually through a chemical bond. Molecules of noble gases (Group 17 of modern periodic table) are monoatomic in nature like He, Ne and Ar etc. Some molecules are diatomic in nature like H2, O2 and N2. Ozone (O3) is an example of triatomic molecule. Molecules having more than three atoms are generally regarded as polyatomic molecules. For example, NH3(Ammonia) is a polyatomic molecule consisting of one nitrogen and three hydrogen atoms.

That's all from me for today's lecture. I tried my best to give you an understanding about atom and molecule in simple words. I have already made an introductory post as chemistry teacher in this community. The link to that post is given below.