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HISTORY OF THE MODERN ATOMIC THEORY Chapter 3 - Sections 3-1 and 3-2 Greek Philosophers ◦ Democritus (400 B.C. ) – All matter is made of basic particles which are indivisible and are called “atomos” . Aristotle (400 B.C.) – All matter is made of earth, fire, wind, and water. Aristotle’s theory of earth, wind, fire, and water was believed until the early 1800’s. Dalton’s Atomic Theory - 1808 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements can combine in simple, whole-number ratios to form chemical compounds. 5. Chemical reactions change the arrangement of atoms, but do not change atoms of one element into atoms of another element. Dalton’s Atomic Theory Not all of the points of Dalton’s theory are correct: 2 – Dalton did not know about isotopes. 3 – Dalton did not know about subatomic particles. Discovery of the Electron 1897 J.J. Thomson (British) is credited with the discovery of the electron Used cathode ray tube and magnets 1907 R.A. Millikan (University of Chicago) Mass of electron = 1/1837 mass of proton Prediction of Proton ◦ 1913 - J.J. Thompson found a negative particle and predicted the discovery of the proton a positive particle to make the atom neutral ◦ JJ Thompson- Plum pudding model ◦ Protons and electrons are arranged randomly in the atom. Thompson’s Model of the Atom Discovery of the Nucleus ◦1911 Lord Ernst Rutherford Gold foil experiment Found that most of the atom was empty space but there was a dense, positive area in the middle that he called the nucleus. Rutherford shot alpha particles at a piece of gold foil. Most of the particles passed through the gold foil without deflection and only a small percentage bounced off the gold foil at a very large angle Discovery of the Proton ◦1919 Ernst Rutherford is credited with the discovery Found he could make one element into another by hitting the nucleus with a helium nucleus. A hydrogen nucleus was emitted by the process and this was found to be only a proton Discovery of the Neutron ◦1932 Sir James Chadwick Student of Ernst Rutherford The atom was heavier than the total mass of the proton and the electron. The charge of the atom was neutral. There must be a neutral particle about the size of the proton. Bohr Model ◦ Danish physicist Niels Bohr, 1913 ◦ A student of Rutherford, developed a new atomic model. ◦ Added new discoveries about how the energy of an atom changes when the atom absorbs or emits light. ◦ Proposed that an electron is found only in specific circular paths, or orbits, around the nucleus ◦ Discovered a Hydrogen atom absorbed light when an electron is excited from a low energy orbit into a higher energy orbit. Atoms can give off light when an electron drops from the high to the low energy orbit. Bohr Hydrogen Experiment Quantum Mechanical Model◦ Current model of the atom ◦ In the Bohr model of the atom, electrons move in circular orbits around the nucleus. ◦ Scientists now know electrons DO NOT travel in orbits around the nucleus. ◦ However, in our currently accepted model of the atom, we only know the probability of finding an electron in an electron cloud, which is a region of space around the nucleus. Aristotle400BC Said all matter is made earth, fire, wind, and water DemocritusJohn Dalton400BC 1808 Said all Came up matter is with 5 made of postulates to “atomos” the atomic theory. 1897- JJ Thomson discovered electron using cathode ray tube 1900- JJ Thomson’s plum pudding model. Protons and electrons are arranged randomly 1911Rutherford discovered the dense (+) charged nucleus- Gold Foil Experiment 1919Rutherford discovered the proton 1922Niels Bohr Model of atom 1932 Chadwick discovered the neutron 1940’s– Quantum Mechanic Model of atom EXIT TICKET List the following scientists in order of their contributions to the structure of an atom. ____ Niels Bohr ____ James Chadwick ____ John Dalton ____ Democritus ____ Ernst Rutherford ____ J. J. Thompson The Atom ◦Nucleus ◦ Proton- positively charged ◦ Neutron- neutral ◦Outside the nucleus- Clouds ◦ Electron- negatively charged Subatomic particles Particle Charge Symbol Electron -1 e Proton +1 p+ 1/2000 the Outside mass of nucleus proton Same mass In nucleus as neutron Neutron 0 n Same mass In nucleus as proton - Mass Location Important Definitions ◦ Atomic Number (Z) – The number of protons in the nucleus ◦ Mass #= protons + neutrons ◦ Electrons= atomic # ◦ Neutrons= mass # - atomic # Ions- Loss or gain of electrons Cation= + charged particle, lost an eAnion= - charged particle, gained an e- Element Symb ol Calcium ion 267 P3- 15 31 Ca+2 20 40 33 75 82 50 19 39 26 56 16 32 Arsenic Tin Potassium ion K+1 Iron Sulfur ion # of # of Mass Electrons Neutrons Number 104 Rutherfordium Phosphorus ion Atomic Atomi # of numbe c mass Protons r S-2 Day 2 Important Definitions ◦ Atomic Mass – The weighted average of the mass numbers of all of the naturally occurring isotopes of an element. Important Definitions ◦ Mass Number – The total number of protons and neutrons in the nucleus of an isotope. # p+ Isotope Mass number Protium Deuterium Tritium 1 1 1 1 1 1 # e- #n 0 1 2 1 2 3 Important Definitions ◦ Isotope – Two or more atoms of the same element that have the same number of protons and electrons but a different number of neutrons. Nuclide – A general term for an isotope of any element. Every element has at least one isotope and some elements have many isotopes General Format of Isotopes ◦ Hydrogen -3 ◦ Oxygen-17 ◦ Lithim-5 ◦ Carbon-12 Hyphen Notation ◦ Write the general and hyphen notation General Format Atomic #= 2 Mass # = 4 Atomic #= 16 Mass # = 32 Atomic #= 17 Mass # = 37 Hyphen Notation Isotop e Zn-64 Zn-66 Zn-67 Gener Number Number Number al of of of format protons electron neutrons s Mass Number Which of the following pairs are isotopes? A. C. B. D. 40 X 20 42 X 20 Day 3 Isotopes of chlorine ◦ Chorine-35 accounts for about 75 percent of the naturally occurring chlorine atoms; chlorine-37 accounts for only about 25 percent. Natural percent abundance Natural Percent Abundance of Stable Isotopes of Some Elements Important Definitions ◦ Atomic Mass Unit – Exactly 1/12 the mass of a carbon-12 atom. Symbol = amu Examples: Mg = 24.31 amu N = 14.01 amu Round off to two places past the decimal Atomic Mass Practice Copper consists of 69.17% Cu-63 and 30.83% Cu-65. Determine the atomic mass of Cu. 63 x .6917 = 43.58 amu 65 x .3083 = 20.04 amu Atomic Mass = 63.62 amu It’s your turn to practice ◦Neon is composed of three naturally occurring isotopes: ◦90.92% is Neon-20, ◦0 .257% is Neon-21 and ◦8.82% is Neon-22. Calculate the atomic mass of neon. Solving the Problem ◦ 0.9092 x 20 = 18.184 ◦ .00257 x 21 = .05397 ◦ .0882 x 22 = 1.9404 ◦ Sum ◦ Answer = 20.1783 = 20.18 amu THAT’S ALL FOR TODAY