Early Atomic Theory Chapter 3 Democritus 460 BC- ~ 370 BC Nothing exists except atoms and empty space; everything else is opinion. Matter is composed of small indivisible particles, atomos meaning Indivisible Plato and Aristotle didn t buy it (play-doe Vs legos) The Atom Aristotle Matter was continuous. Did not believe in atoms 4 elements fire, air, earth and water Blend all 4 of these in different proportions to get all substances Neither theories supported by experimental evidence Alchemy 2000 years of Chemical History Many were scientists & made important contributions to modern Chemistry Some looked to turn metals into gold Important contributions: Discover of Mercury, Sulfur and Antimony; lab procedures and preparing acids. Atom The atom The smallest unit of an element that retains all the properties of that element 1
Foundations of Atomic Theory Law of conservation of mass (Matter) Mass is neither created nor destroyed during ordinary chemical reactions or physical changes (Antoine Lavoisier, 1774) 16 X + 8 Y 8 X 2 Y 2.1 Foundations of Atomic Theory Law of Definite Proportions (Proust) Chemical compounds contain the same elements in exactly the same proportion regardless of sample size NaCl Sodium Chloride 39% Sodium and 61% Chlorine Water will always contain 8g of Oxygen for every 1g of Hydrogen Dalton s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. 2. All atoms of a given element are different from the atoms of all other elements. 3. Compounds are composed of atoms of more than one element in fixed ratios. 4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction of atoms. 2.1 Law of Multiple Proportions If two or more different compounds are composed of the same two elements, the ratio of the masses of elements are always a ratio of small whole numbers. CO, CO 2, CO 3 2 2.1 2
Example of Law of Multiple Proportions Carbon combines with oxygen to form CO and CO 2. Mass of Carbon(g) Mass of Oxygen(g) CO 12.01 16.00 CO 2 12.01 32.00 2:1 Ratio of O in CO 2 to O in CO Mysterious world of the atom Let s probe deeper Electron -properties Model of the atom Experiments to probe atom: + Cathode ray tube + Mulliken Oil drop + Rutherford scattering Crookes Tube 1869-1875 Something was making a glowing shadow Parts of the Atom Discovery of Electron J. J. Thomson English Physicist (1897) Made a piece of equipment called a cathode ray tube Adaptation of Crookes tube Cathode Ray experiment Discovery of the electron Experiments confirmed that electron has negative charge CATHODE RAY TUBE EXPERIMENT Thomson s Experiment - Voltage source + J.J. Thomson, measured mass/charge of e - Vacuum tube Metal Disks (1906 Nobel Prize in Physics) 2.2 3
Thomson s Experiment Thomson s Experiment - Voltage source + Voltage source + n Passing an electric current makes a beam appear to move from the negative to the positive end n By adding an electric field he found that the moving pieces were negative - Thomson Summary Found the electron! Couldn t find positive (for a while) Said the atom was like plum pudding A bunch of positive stuff, with the electrons able to be removed Robert Millikan Robert Millikan Oil Drop Experiment Electron mass 9.109 * 10-31 kg Electron Charge 1.602 * 10-19 Coulomb Relative charge (-1) Millikan s Oil Drop Experiment Measured charge and mass of e - (1923 Nobel Prize in Physics) 2.2 Rutherford s experiment Used radioactivity Alpha particles - positively charged pieces given off by uranium (2 protons & 2 neutrons) Shot them at gold foil which can be made a few atoms thick When the alpha particles hit a florescent screen, it glows. 4
Lead block Uranium Florescent Screen THOMSON (AND KELVIN) S PLUM PUDDING MODEL OF THE ATOM Gold Foil He Expected The alpha particles to pass through without changing direction very much Because (he thought) the positive charges were spread out evenly. Alone they were not enough to stop the alpha particles GOLD FOIL EXPERIMENT What he expected: Because, he thought the mass was evenly distributed in the atom What he got 5
How he explained it Atom is mostly empty Gold Foil experiment Small dense, positive piece at center Alpha particles are deflected by it if they get close enough + + Discovery of Atomic Nucleus Rutherford s Gold foil experiment Concluded atom had a positively charged nucleus Volume of nucleus small compared to volume of atom Most of the mass of the atom contained in nucleus (dense) Composition of Atomic Nucleus Proton positive charge Number of protons in nucleus determines atoms identity Atomic Number Mass 1.673 * 10-27 kg Relative Charge (+1) Neutron neutral no charge Mass 1.675 * 10-27 kg Summary of Subatomic particles Structure of the atom Name Symbol Charge Electron e - -1 Relative mass 1/1840 Actual mass (g) 9.11 x 10-28 Atoms composed of smaller particles Two regions Nucleus Protons and Neutrons Proton Neutron p + n 0 +1 0 1 1 1.67 x 10-24 1.67 x 10-24 Electron Cloud Number and arrangement of these particles determine chemical properties Neutral charges add up to zero 6
Atoms are small. Size of an atom Measured in picometers, 10-12 meters ex: Hydrogen atom, 32 pm radius Nucleus tiny compared to atom If the atom were the size of a stadium, the nucleus would be the size of a marble. Radius of nucleus ~ 10-15 m Density near 10 14 g/cm Counting the Pieces Atomic Number = number of protons # of protons determines kind of atom the same as the number of electrons in the neutral atom Mass Number = the number of protons + neutrons All the things with significant mass Isotopes Atoms of the same element (same # of protons) can have different numbers of neutrons different mass numbers called isotopes Ions Elements can also gain or lose electrons. Each electron has a -1 charge so; Adding an electron = -1 Removing an electron = +1 An element with a charge (gained or lost electrons) is an ion.. Cations & Anions Cations are ions with positive charges Lost one or more electrons Ca+ion Think cats are nice (positive) but always looking around for their electron Anions are ions with negative charges Gained one or more electrons Nuclear Symbols Contain the symbol of the element, the mass number and the atomic number Mass number Atomic number X Charge (ion only) 7
Nuclear Symbols Find the number of protons number of neutrons number of electrons Atomic number Mass Number Cation, Anion or Neither? 19 9 F Nuclear Symbols n Find the number of protons number of neutrons number of electrons Atomic number Mass Number Cation, Anion or Neither? 80 35 Br -1 Symbols n If an element has an atomic number of 34 and a mass number of 78 what is the? number of protons number of neutrons number of electrons Complete symbol Symbols n if an element has 91 protons and 140 neutrons and a +2 charge what is the? Atomic number Mass number Number of electrons Complete symbol Symbols n if an element has a +1 charge, 78 electrons and 117 neutrons what is the? Atomic number Mass number number of protons Complete symbol Naming Isotopes Hyphen Notation Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235 8
Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each isotope in nature. Don t use grams because the numbers would be too small Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) Defined as one twelfth the mass of a carbon- 12 atom. Each isotope has its own atomic mass we need the average from percent abundance. Calculating averages A student has an 81 test average, a 89 quiz average, a 90 lab average, and 100 homework average. Average grade is? 81+ 89 + 90 + 100 = 360 360 = 4 90 average! Calculating weighted averages Tests = 50%; Quizzes 20%; Labs 15 %; Homework 15% Weighted average = (.5 x 81) + (.2 x 89) + (.15 x 90) + (.15 x 100) = Weighted average = 40.5 + 17.8 + 13.5 + 15 Weighted average = 86.8 (rounds to 87) Atomic Mass Boron has 2 isotopes. 20% is Boron-10, and 80% is Boron-11. What is the average atomic mass? (.20 X 10)+(.80 X 11) =? 10.8 amu Check out the periodic table! Atomic Mass Calculate the atomic mass of lead if 1.37% is lead-204, 26.26% is lead-206, 20.82% is lead- 207, and 51.55% is lead 208. 207.21 amu If not told otherwise, the mass of the isotope is the mass number in amu 9
Atomic Mass Is not a whole number because it is an average. The average is based on the how often each isotope appears in nature Explains the decimal numbers on the periodic table. 10