An atom is the basic unit of matter and the building block of all substances. It consists of a nucleus, which contains positively charged protons and neutral neutrons, surrounded by negatively charged electrons that orbit the nucleus. Atoms combine in various ways to form molecules, which make up everything from air to water to living organisms. Atoms are classified into different elements based on the number of protons they contain. The periodic table organizes these elements, such as Hydrogen, Oxygen, and Carbon, each with unique properties. Understanding atoms is fundamental to the fields of chemistry and physics, as they explain the behavior of matter.