Atoms are the basic building blocks of all matter and are made up of a nucleus containing protons and neutrons surrounded by electrons. John Dalton's atomic theory established that atoms are indivisible and that different elements are composed of different types of atoms. Rutherford discovered that the nucleus contains a concentrated positive charge and most of an atom's volume is empty space. Niels Bohr's model represented electrons orbiting the nucleus in fixed energy levels like planets around the sun, explaining atoms' distinct properties and how they form bonds.