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Atomic Theory The History of the Atom What do I need to know? How did the following people impact the development of the atomic model? Democritus John Dalton JJ Thomson Ernest Rutherford James Chadwick Sir William Crookes Niels Bohr Democritus A contemporary of Aristotle around 400 BC. The world was made up of two things: 1. empty space 2. tiny particles called “atomos”. Isaac Newton and Robert Boyle Lived during the late1600s. Spoke out in support of Democritus. No experimental evidence of this belief. John Dalton English schoolmaster and chemist Lived in the 1800s Called “The Father of the Modern Atomic Theory” because he offered the first rational atomic theory based on the scientific evidence of the time. John Dalton’s Atomic Theory Supported Democritus’ idea of the atom. Was the first atomic theory based on scientific evidence. Combined ideas of elements with that of atoms Published in 1808 Scientific Evidence available to Dalton 1 Antoine Lavoisier French Chemist Total mass before chemical reaction was the same as the total mass after chemical reaction. Mass of reactants = Mass of products Law of Conservation of Mass Antoine For Lavoisier a closed system, the total mass before chemical reaction is equal to the total mass after chemical reaction. Total mass of the universe is a constant Matter can be changed in many ways, but it cannot be created or destroyed (for non nuclear processes) Scientific Evidence Available to Dalton 2 Joseph Proust French Chemist Observed that specific substances always contain elements in the same ratio by mass. Law of Definite Proportions (#3) Each compound has a specific ratio of elements It is a ratio by mass Water is always 8 grams of oxygen for each gram of hydrogen Law of Definite Proportions Joseph Proust Substances always contain elements in the same ratio by mass. Example: For NaCl, for a 100.0 g sample, it will always have 39.3 g Na and 60.7 g Cl i.e 39.3 % Na and 60.7 % Cl John Dalton Armed with this experimental work, John Dalton proposed his atomic theory He only had 2 pieces of experimental work when he made his proposal although other evidence shortly followed (within 3-4 years) He kept Democritus’ idea of particulate matter Dalton’s Atomic Theory (1808) An element is composed of extremely small, indivisible particles called Atoms All of the atoms of a given element have identical properties, which differ from the properties of the other elements. Atoms cannot be created, destroyed, or transformed into atoms of another element. Experiment 1 Law of Conservation of Mass Compounds are formed when atoms of different elements combine with each other in small wholenumber ratios. Law of Multiple Proportions (Dalton proposal not based on experiment but proven later to be true by Amadeo Avagadro and J.L. Gay-Lussac) The relative numbers and kinds of atoms are constant in a given compound. Experiment 2 Law of Definite Proportions Law of Multiple Proportions CO or CO2, but not CO1.8 Law of Multiple Proportions if two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number. What? Water is 8 grams of oxygen per gram of hydrogen. Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. 16 to 8 is a 2 to 1 ratio True because you have to add a whole atom, you can’t add a piece of an atom. Subatomic Theory in the 1800s Sir William Crookes JJ Thomson Millikan Rutherford Summary of Dalton’s Atomic Theory 1. 2. 3. 4. It was based on experiment It explained the concepts of atoms, elements, and compounds It explained why there were differences b/w the properties of the elements. Included the three foundational laws of chemistry (LCM, LDP, LMP) Discovery of the Electron 1897 It required to two separate experiments by two different individuals: JJ Thomson and Robert Millikan JJ Thomson and the Cathode Ray Tube (CRT)—1897 JJ Thomson used the CRT to measure the charge-to-mass ratio (charge : mass) of a kg of electrons. He won the 1906 Nobel Prize in Physics for this work. Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of different elements are different. Atoms of different elements combine in whole number ratios to form compounds In Chemical reactions, atoms are rearranged, combined or separated. No new atoms are created or destroyed. J. J. Thomson English physicist 1897 Used the cathode ray tube It is a vacuum tube - all the air has been pumped out. Couldn’t measure the charge directly so he determined the ratio of charge to mass. Thomson’s Experiment CATHODE Voltage source - ANODE + Vacuum tube Metal Disks Thomson’s Experiment Voltage source - + Thomson’s Experiment Voltage source - + Thomson’s Experiment Voltage source - + Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field Thomson’s Experiment Voltage source + By adding an electric field he found that the moving pieces were negative CRT Expt Next, he placed an object in the path of the CR. Part of the beam was blocked and… Cathode (-) This proved conclusively that the CR was made of particles. Anode (+) A shadow was created on the anode. CRT Expt Finally he placed a paddle wheel in the path of the CR—the paddle wheel spun. Cathode (-) Anode (+) This proved that the particles that made up the CR had mass. CRT Expt Summary Thomson showed: 1. The CR was charged 2. The CR had a negative charge 3. The CR was made of particles 4. The CR particles had mass. ** The deflection of the cathode rays gave evidence for the negatively charged nature of electrons. ** What does it really look like? CRT Expt Summary Thomson won the Nobel prize because he measured the charge : mass ratio of a kg of electrons. He measured this to be 1.759 X 1011 C/kg and the charge was negative (electrons are negatively charged). Contributions… WHO? Democritus DID WHAT? Proposed idea of Atoms Sir William Crookes Discovered cathode ray (charged particles) J.J. Thomson Discovered Electron charge to mass ratio PROBLEM? What holds atoms together? Unable to determine exact charge. Millikan / Thomson Result With the results of these 2 experiments, the mass of the electron could then be calculated. Thomson: Millikan: 1.759 X 1011 C / kg 1.6 X 10-19 C / e- Today, the electron is the standard unit of negative charge Thomson’s Model / Plum Pudding Found the electron Couldn’t find positive Said the atom was like plum pudding Positive & negative charges evenly spread through out, with the electrons able to be removed Rutherford’s experiment (Aware of Thomson’s plum pudding Used radioactivity Alpha particles - positively charged pieces given off by uranium Shot them at gold foil which can be made a few atoms thick The modern view of the atom was developed by Ernest Rutherford (1871-1937). He Expected The alpha particles to pass through without changing direction very much Because… The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles Results of foil experiment if Plum Pudding model had been correct. What he expected He thought the mass was evenly distributed in the atom Because, he thought the mass was evenly distributed in the atom What he got How he explained it Atom is mostly empty Small dense, positive center Alpha particles are deflected by it if they get close enough + + What Actually Happened Rutherford’s experiment. Plum Pudding Model must be Incorrect YOU MUST KNOW THIS…WRITE THE FOLLOWING DOWN… Concluded: Atom is mostly empty space Center (nucleus) Positive particles and most of the atom’s mass will be found here. Nucleus is DENSE Structure of the Atom There are two regions The nucleus With protons and neutrons Positive charge Almost all the mass Electron cloud- Most of the volume of an atom The region where the electron can be found Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloudregion where you might find an electron Subatomic particles Relative Actual mass (g) Name Symbol Charge mass Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 0 1 1.67 x 10-24 Contributions… WHO? Millikan Rutherford DID WHAT? Accurately determined • Charge (-1) • Mass of electron (1/1840 the mass of Hydrogen atom) Problem – couldn’t account for all of the mass. Concluded • Discovered Proton • Nucleus is positive • Nucleus is dense • Atom mostly empty space Contributions… WHO? Neils Bohr DID WHAT? Electrons orbit nucleus like planets “planetary model” Electrons travel in fixed orbits James Chadwick (Rutherford’s lab assistant) Discovered Neutron click here for video on discovery Atomic Number Moseley found that different elements have different numbers of protons. In fact, he discovered that elements had incrementally different numbers of protons Called this number the Atomic Number Discovery of the Proton Atomic Number tells us: 1. Number of protons in an atom 2. Identity of the element (Au, Ag, etc) Implications for Dalton’s Atomic Theory The discovery of the proton, electron, and the neutron was the first major contradiction to Dalton’s Atomic Theory Dalton’s Atomic Theory postulated that the atom was the smallest piece of matter—we now know this not to be true. Subatomic Particle Summary (pg 111) Particle Actual Charge Relative Charge Electron -1.9 X 10-19 C Proton Neutron +1.9 X 10-19 C 0 -1 +1 0 Subatomic Particle Summary * KNOW Relative Mass * Particle Actual Mass Relative Mass Electron 9.10939 X 10-31 kg 0 1.67262 X 10-27 kg 1 Neutron 1.67493 X 10-27 kg 1 Proton Subatomic Particle Summary SO WHAT DO I NEED TO KNOW? Test on Tue. (10/7) You need to remember: --Who discovered what and how (Thomson & Millikan, Crookes, Rutherford, Bohr & Chadwick) --The following in this table: Particle Electron Proton Neutron Relative Mass 0 1 1 Relative Charge -1 +1 0 Symbols Contain the symbol of the element, the mass number and the atomic number Mass number Atomic number X Mass Number Number of protons + number of Neutrons Atomic Number The number of protons in the nucleus of an atom. Find the number of electrons and protons contain in an atom of: A. B. C. D. E. F. 33 p+ Arsenic Gold 79 p+ Fluorine 9 p+ Molybdenum 42 p+ Polonium 84 p+ Barium 56 p+ 33 e- 79 e9 e42 e84 e56 e- Identify the atom containing the following number: A. B. C. D. E. 34 5 31 61 94 Selenium Boron Gallium Promethium Plutonium Identify the atom containing the following number of protons: A. 74 Tungsten B. 20 Calcium C. 49 Indium D. 70 Ytterbium E. 93 Neptunium Nuclear Symbol and PEN diagrams P protons E electrons N neutrons PEN diagram is nothing more than a diagram that tells how many protons, electron, and neutrons are in an atom. Nuclear Symbols and PEN diagrams Mass Number (A): # protons + # neutrons Atomic Number (Z): # protons Nuclear Symbol: symbol that designates the specific atomic number and mass number of a nuclide. Nuclear Symbol and names 37Cl is the same as chlorine-37 15N is the same as nitrogen-15 PEN Example Copper-65 65Cu P E N 29 29 36 A – Z = # neutrons from PT if neutral, same as Z A-Z Your turn – class work – complete for homework Practice Atomic Structure Worksheet#1 and #2 Isotopes and Atomic Mass While scientists were discovering the subatomic particles, other scientists were involved with isolating and characterizing the naturally occurring elements. These scientists were having difficulty understanding their mass measurements especially considering the now widely accepted fact of subatomic particles Isotopes and Atomic Mass Scientists determined that the atomic mass was actually a weighted average of the naturally occurring isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons This gives atoms of the same element different masses and the measured atomic mass of an element is then a weighted average of the naturally occurring isotopes. Isotopes and Atomic Mass Mass Number: The number of protons and neutrons in an isotope of an atom Atomic Mass: Weighted average of all of the naturally occurring isotopes of an element. What’s the standard for the mass of an atom? STANDARD IS THE CARBON – 12 ATOM The Carbon-12 atom was assigned a mass of exactly 12 atomic mass units (amu) ATOMIC MASS UNIT amu = 1/12 of a carbon 12 atom Although 1 amu is approximately equal to 1 proton or 1 neutron there are differences Subatomic Particles Masses (p.119) KNOW Electron = 0.000549 = 0 amu Proton = 1.007276 = 1.00 amu Neutron = 1.008665 = 1.00 amu Isotopes and Atomic Mass What is a weighted average? Consider your grade: 40 % Daily 60 % Tests Avg. = 80? 70 90 Isotopes and Atomic Mass Not if it’s a weighted average. The test grade is weighted more heavily so the average should reflect this. 40 % Daily 60 % Tests 70 X .40 = 90 X .60 = 28 54 28 + 54 = 82 Avg. = 82 Atomic Mass Atomic mass is a weighted average. To calculate the atomic mass - you need the following to be given to you: relative abundance of the isotopes mass of the isotope. Steps to determine atomic mass: Step 1: Convert the relative abundance (%) into decimal format. Step 2: Multiply the decimal from step 1 by the mass of that isotope. Step 3: Do this for each isotope. Step 4: Add the results of all isotope calculations This will provide you with the atomic mass. Let’s take a look… Atomic Mass Calculation Isotope Rel. Abundance Mass (amu) 16O 18O 99.759 % 0.037 % 0.204 % 15.995 amu 16.995 amu 17.999 amu At. Mass = 17O 0.99759(15.995) + 0.00037(16.995) + 0.00204(17.999) = 15.9995 = 16.00 amu Bellringer (10/6/08) Take out Atomic Mass Worksheet On a separate sheet of paper answer the following: 1. 2. An ion always contains: A. equal number of p+ and eB. equal number of p+ and n0 C. Unequal number of p+ and eD. Unequal number of n0 and eWhat is the approximate mass of a neutron in amu’s? Let’s Review from Friday… Measuring Atomic Mass Unit is the Atomic Mass Unit (amu) One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we need the average from percent abundance. Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235 How do isotopes Hydrogen – 1 and Hydrogen -2 Differ ? H – 2 has one neutron and H – 1 has no neutrons Atomic Mass are the decimal numbers on the periodic table. Is not a whole number because it is an average. Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. (.691) (62.93 amu) = 43.48 100% - 69.1% = 30.90 % (.3090) (64.93 amu) = 20.06 43.48 + 20.06 = 63.54 amu Check your work – what is the atomic mass for Copper? Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu Need more Practice / information see p. 119-121 See top of my web page for all downloads for this unit.