The Model of the Atom The model of the atom is built on evidence. The model changes as more evidence is discovered.

What is an atom? Atom – “indivisible” – fundamental unit of matter (even though we know about subatomic particles) An atom is the smallest representative of an element that has properties of that element. An atom contains protons and neutrons in the nucleus and electrons outside the nucleus.

The Philosophers 600 B.C. Thales pondered what matter is made of; discovered static electricity. 430 B.C. Democritus – Matter can be broken into smaller and smaller pieces until the smallest piece of matter is reached. He called the small pieces of matter “atomos” meaning indivisible. 384-322 B.C. Aristotle claims matter is made up of 4 elements: earth, fire, wind, and water.

The Classical Period 1600s Alchemy became popular. 1610 Galileo as an experimenter proved that all objects fall at the same speed. Aristotle had stated that the speed of the falling object depended on its weight. He proved Aristotle wrong, and people began to question Aristotle’s matter teachings 1752 Ben Franklin discovered + and – charges. Opposites attract and like charges repel.

The Classical Period 1789 Antoine Lavoisier established the Law of Conservation of Matter (Mass). No mass is gained or lost in a chemical reaction.

The Classical Period 1803 John Dalton established the Law of Multiple Proportions. The mass ratios of elements in a compound can be expressed with whole #’s. properties of matter can be explained in terms of atoms The Atomic Theory of Matter has 5 postulates. (p. 89)

The Classical Period Dalton’s Atomic Theory All matter is composed of small particles called atoms. All atoms of a given element are identical to each other but different from atoms of other elements. Atoms cannot be created, divided, or destroyed. Different atoms combine in simple whole-number ratios to form compounds. In a chemical reaction, atoms are rearranged.

The Classical Period 1811 Count Amedeo Avogadro realized some elements are diatomic. He discovered molecules. 1839 Michael Faraday said the structure of the atom is related to electricity. (Atoms contain particles that have electrical charge.) 1879 William Crookes invented the Cathode Ray Tube (Crooke’s Tube) and noticed that particles moved from the negative to the positive electrode.

The Classical Period 1885 Eugene Goldstein found positive particles (protons) when using a Crooke’s tube filled with hydrogen gas 1895 Willhelm Roentgen discovered x-rays 1896 Henri Becquerel discovered that pitchblende (a mineral ore) exhibits radioactivity.

The Classical Period 1897 J. J. Thomson realized that when exposed to a magnetic field the particles of the cathode ray were attracted to a positive pole and must be negatively charged; called these particles electrons. determined the charge to mass ratio of electrons postulated that the atom must have a positively charged particle to balance with electrons making the atom neutral developed the “plum pudding” model of the atom: a ball of positive charge with negatively charged electrons embedded inside

The Classical Period 1897 Marie Curie isolated the elements in pitchblende and found that some elements radiated energy. She coined the term radioactivity. 1900 Max Planck said that energy is absorbed or given off in discrete bundles called quanta. This is the beginning of the Quantum Theory

The Classical Period 1905 Albert Einstein proposed that electromagnetic radiation can behave like waves and like particles called a particle of electromagnetic radiation a photon; it carries a quantum of energy Published Theory of Relativity (E = mc2)

The Classical Period 1909 Robert Millikan determined that an electron is about 2000x lighter than a H atom 1909 Robert Millikan measured the charge of an electron by bombarding oil drops with x-rays to make them negative and measuring the rate that the drop fell based on charges of electric plates around them charge of each drop was a multiple of 1.60x10-19C

The Classical Period 1911 Ernest Rutherford bombarded a thin layer of gold foil with alpha (+) and beta (-) particles in “gold foil experiment.” a few + particles were deflected (some straight back) proposed that an atom has a very small but dense positive nucleus and much empty space and that electrons orbit the nucleus realized there were 3 types of radioactivity with different penetrating power

The Modern Period 1913 Niels Bohr saw a problem with Rutherford’s model. Electrons would be attracted to the nucleus and collide with it after losing all energy. He proposed that electrons could only stay in certain energy levels. Levels closest to the nucleus have the lowest energy. Electrons must give off a certain amount of energy to move from an outer to an inner level. Electrons must gain a certain amount of energy to move from an inner to an outer level. That certain amount of energy is called a quantum.

The Modern Period 1914 Henry Moseley used x-rays to determine the number of protons in each atom. 1924 Louis-Victor de Broglie said particles such as electrons exhibit wave properties. Electrons are only allowed certain wavelengths and energies. 1926 Werner Heisenberg found that the exact momentum and exact position of an electron cannot be known simultaneously. Measuring wave properties disturbs the particle properties. Heisenberg’s Uncertainty Principle

The Modern Period 1926 Erwin Schroedinger introduced an equation that is used to determine the probability of finding an electron within a particular volume of space around the nucleus. He put ideas of deBroglie and Heisenberg together. 1928 Wolfgang Pauli showed that no 2 electrons can occupy the same space unless their magnetic fields (spins) are opposite in the Pauli Exclusion Principle; reason chemistry of atom depends on electron configuration In 1 dimension

The Modern Period 1932 Enrico Fermi bombarded atoms with neutrons to create atoms with higher atomic numbers. When he bombarded a uranium atom, it split. He discovered nuclear fission. 1932 James Chadwick discovered the neutron 1934 Irene Joliot-Curie produced radioactive substances artificially by bombarding certain elements with alpha particles.