The smallest unit of matter is called an "atom." Atoms consist of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. These fundamental building blocks combine to form molecules, which make up everything we encounter in our daily lives, including solids, liquids, and gases. Atoms are incredibly tiny, with a standard size on the order of picometers. They display various properties based on their type, such as elements in the periodic table. Understanding atomic structure is crucial in fields like chemistry and physics, underpinning the behavior of matter.