An atom is the smallest unit of a chemical element and consists of a dense, positively charged nucleus at its center, containing protons and neutrons, surrounded by a cloud of negatively charged electrons orbiting in shells. Protons and neutrons constitute the vast majority of an atom's mass, while the number of protons, known as the atomic number, defines the element. Atoms are the building blocks of all matter, and their arrangement determines the physical and chemical properties of substances