2Chemistry Timeline #1 2000 years of Alchemy B.C. 400 B.C. Demokritos and Leucippos use the term "atomos” years of Alchemy1500'sGeorg Bauer: systematic metallurgyParacelsus: medicinal application of minerals1600'sRobert Boyle:The Skeptical Chemist. Quantitative experimentation, identification ofelements1700s'Georg Stahl: Phlogiston TheoryJoseph Priestly: Discovery of oxygenAntoine Lavoisier: The role of oxygen in combustion, law of conservation ofmass, first modern chemistry textbook
3Chemistry Timeline #21800'sJoseph Proust: The law of definite proportion (composition)John Dalton: The Atomic Theory, The law of multiple proportionsJoseph Gay-Lussac: Combining volumes of gases, existence of diatomic moleculesAmadeo Avogadro: Molar volumes of gasesJons Jakob Berzelius: Relative atomic masses, modern symbols for the elementsDmitri Mendeleyev: The periodic tableJ.J. Thomson: discovery of the electronHenri Becquerel: Discovery of radioactivity1900'sRobert Millikan: Charge and mass of the electronErnest Rutherford: Existence of the nucleus, and its relative sizeMeitner & Fermi: Sustained nuclear fissionErnest Lawrence: The cyclotron and trans-uranium elements
4Law of Definite Proportion Different samples of a pure compound always contain the same elements in the same proportions by mass.Water, for example, always has an 8:1 proportion of Oxygen to HydrogenTypically, this is written as a mass percent, so we say that water is 88.9% Oxygen (by mass) – what percent by number is this?16 amu Oxygen2 amu Hydrogen32 amu Oxygen4 amu Hydrogen48 amu Oxygen6 amu Hydrogen
5Law of Definite Proportion Examples What Mass of Oxygen is contained in a 50.0 g sample of water?44.5 g OWhat mass of Hydrogen is in a 35.5 g sample of water?3.94 g HAn unknown water like substance was broken down and found to be composed of 58.8 g of Oxygen and 3.7 g Hydrogen. Could this substance be water? Show a calculation to support your answer.No (shown to be 94.1 % Oxygen)
6Law of Multiple Proportion If two elements combine to make different compounds, keeping one elements mass constant will always result in a whole number ratio of the second element.50 grams of carbon can combine with 66.5 g of oxygen or 133 g of oxygen. What is the ratio of oxygen to oxygen in each compound?14 grams of Nitrogen can combine with 8, 16, 24, 32 or 40 g of oxygen to form 5 different compounds. What is the ratio of oxygen in each?
7Dalton’s Atomic Theory (1808) All matter is composed of extremely small particles called atomsAtoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other propertiesJohn DaltonAtoms cannot be subdivided, created, or destroyedAtoms of different elements combine in simple whole-number ratios to form chemical compoundsIn chemical reactions, atoms are combined, separated, or rearranged
8Modern Atomic TheorySeveral changes have been made to Dalton’s theory.Dalton said:Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other propertiesModern theory states:Atoms of an element have a characteristic average mass which is unique to that element.
9Modern Atomic Theory #2 Dalton said: Atoms cannot be subdivided, created, or destroyedModern theory states:Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions
10Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.The rays showed that the charge to mass ratio of the negative particles was always the same, no matter the gas used.
11Thomson’s Atomic Model J.J. ThomsonThomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
12Mass of the Electron1909 – Robert Millikan determines the mass of the electron.Mass of the electron is9.109 x kgThe oil drop apparatus
13Conclusions from the Study of the Electron Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electronsElectrons have so little mass that atoms must contain other particles that account for most of the mass
14Rutherford’s Gold Foil Experiment Alpha particles are helium nucleiParticles were fired at a thin sheet of gold foilParticle hits on the detecting screen (film) are recorded
15Rutherford’s Findings Most of the particles passed right throughA few particles were deflectedVERY FEW were greatly deflected“Like howitzer shells bouncing off of tissue paper!”Conclusions:The nucleus is smallThe nucleus is denseThe nucleus is positively charged
16Atomic Particles Particle RelativeCharge Mass (kg) Location Electron -19.109 x 10-31Electron cloudProton+11.673 x 10-27NucleusNeutron1.675 x 10-27
17The Atomic ScaleHelium-4Most of the mass of the atom is in the nucleus (protons and neutrons)Electrons are found outside of the nucleus (the electron cloud)Most of the volume of the atom is empty space1 million femtometers = 1 nanometer1 million nanometers = 1 millimeterImage: User Yzmo Wikimedia Commons.
18About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark.Neutrons are made of one “up” quark and two “down” quarks.Quarks are held together by “gluons”Images: Arpad Horvath, Wikimedia Commons.
19IsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.IsotopeProtonsElectronsNeutronsNucleusHydrogen–1(protium)1Hydrogen-2(deuterium)Hydrogen-3(tritium)2
20Composition of the nucleus Atomic MassesAtomic mass is the average of all the naturally isotopes of that element.IsotopeSymbolComposition of the nucleus% in natureCarbon-1212C6 protons6 neutrons98.89%Carbon-1313C7 neutrons1.11%Carbon-1414C8 neutrons<0.01%Carbon =
21Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.Element# of protonsAtomic # (Z)Carbon6Phosphorus15Gold79
22Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.Mass # = p+ + n0Nuclidep+n0e-Mass #Oxygen -10-3342- 3115188818Arsenic753375Phosphorus161531
23Calculating Average Atomic Mass Calculate the average atomic mass of Chlorine and Silicon given the following information:ChlorineSiliconMass numberExact massPercent abundance3575.772892.233724.23294.67303.10Chlorine: 35.45Silicon: 28.09