An atom is the basic building block of matter, consisting of a nucleus surrounded by electrons. The nucleus contains positively charged protons and neutral neutrons, while negatively charged electrons orbit around it. Atoms combine to form molecules, which make up everything we see and touch. Atoms are incredibly small, typically measuring around 0.1 nanometers in diameter. Each element on the periodic table is defined by the number of protons in its nucleus, known as the atomic number. This unique structure determines the chemical properties and behavior of the element in various reactions.