Chapter 3
The atom is the basic building block of matter, representing the smallest unit of a chemical element. An atom in turn is composed of subatomic particles called protons, neutrons, and electrons. In 1911, Ernest Rutherford provided experimental evidence that an atom has a dense, positively charged nucleus that accounts for only a small portion of the volume of the atom but nearly all the mass. The protons and neutrons in an atom form the nucleus, the core of the atom. The electrons exist outside the nucleus in characteristic regions of space called orbitals. All atoms of an element show similar chemical properties and cannot be further broken down by chemical means.