History of the Atom

Democritus 400 BC Matter could not be divided forever, you would get to an indivisible piece- called the atom or “uncuttable”. Atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

John Dalton 1803 Experiments with gases showed elements combined to make compounds Elements were made of a unique kind of atom, and atoms of other elements are different. Compounds are formed by the joining of atoms of two or more elements. Billiard ball model- an atom is a small solid sphere

J. J. Thomson 1897 Cathode Ray tube experiment: As an electric current passed through a gas it gave off rays of negatively charged particles. Thomson concluded that the negative charges came from within the atom. So atoms contains smaller charged particles. Plum-pudding model: Atoms were made from a positively charged substance with negatively charged electrons scattered about

Ernest Rutherford 1911 Gold Foil Experiment: hit thin sheet of gold foil with alpha particles, most passed through, but some bounced back Rutherford Model: mass of atom is concentrated in the nucleus atom is mostly empty space dense positively charged nucleus surrounded by negative elections

Niels Bohr 1913 Hydrogen Emission spectrum: hydrogen gas emitted light at specific energy. Bohr Model: Electrons travel around nucleus in specific orbits or energy levels As electrons gain or lose energy they move to a different orbit around the atom Higher orbits have higher energy, lower orbits have lower energy

James Chadwick 1932 While researching atomic decay, noticed the atomic number was less than atomic mass, so there must be more in nucleus besides protons Nucleus is made of protons and neutrons Neutrons are slightly more massive than protons, no charge.

Wave Model: Modern Day Electron Cloud Model: atom has a dense nucleus surrounded by electrons in different clouds at the different energy levels Electrons don’t have a fixed orbit. Clouds are made of orbitals- areas where electrons are likely to be found