3Originated idea of the atom Democritus ( BC)“Atomos” indivisible and indestructibleNo evidence, no experiments for supportPlato and Aristotle argued against him“the properties of matter that we see are explained by the properties and behavior of atoms that we cannot see”Originated idea of the atom
4For the next 2000 years… Alchemy Good or bad? Contributions? PseudoscienceTrying to turn metals into goldDiscovered several elements in the process
5Isaac Newton – 1704 AD“It seems probable to me that God in the beginning, formed matter in solid massy, hard, impenetrable movable particles of such sizes and figures and with such other properties, in such proportions to space, as most conduced to the end for which he formed them.”
61803 Dalton’s Atomic Theory John Dalton ( )Father of Atomic theory, not bad considering he had very little evidence. Controlled experiments, ratios determineStudied the ratios in which elements combine in chemical reactions (used experimental methods)Proposed an atomic theory based on resultsLavoisier –Wrote the first chemistry bookHe was able to explain the true nature of combustion (oxygen) with candles and bell jars.Discovered that mass was conserved in a chemical reaction. (Law of Conservation of Mass)1803 Dalton’s Atomic Theory
7ProustShowed that the proportions by mass of the elements in a given compound are always the same. (Law of Definite Proportions)Conducted quantitative experimentsShowed that regardless of how copper carbonate was prepared in the lab, or how it was isolated in nature, it always contained the same proportions of copper, oxygen, and carbon – 5:4:1 by mass
8At the time, current thinking… George Stahl said that fire went out in a closed container because of phlogiston given off by the fireJoseph Priestly said a substance with heavy combustion is low in phlogiston. Oxygen was dephogisicated air
9Modern Atomic Theory Dalton said: Several 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.
10Modern Atomic TheoryDalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!
11J.J. Thomson (1856-1940) 1897 Discovered the electron (“plum pudding” model)
12Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.Showed cathode ray tube consists of a stream of negatively charged particles he called corpuscles (later called electrons)Applied a magnet tube – electrons moved toward the positive magnet.Concluded negative particlesCathode ray tubes pass electricity through a gas that is contained at a very low pressure.
13Cathode Ray TubeTubeCathodeAlligator ClipAnodeStand
15Mass of the Electron1909 – Robert Millikan determines the mass of the electron.The oil drop apparatusMass of the electron is9.109 x kg
16Conclusions from the Study of Electrons 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 electrons.Electrons have so little mass that atoms must contain other particles that account for most of the mass
18Rutherford (1871-1937) 1911 Discovered the nucleus (gold foil experiment)
19Rutherford’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
20Try it YourselfIn the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?
22Rutherford’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
23Since opposite charges attract each other, why don’t the electrons fall into the nucleus?
241913 proposed Planetary model Niels Bohr ( )1913 proposed Planetary model
25The Bohr Model of the Atom I pictured electrons orbiting the nucleus much like planets orbiting the sun.But I was wrong! They’re more like bees around a hive.WRONG!!!Neils Bohr
26The Quantum-Mechanical Model 1926The Quantum-Mechanical ModelBased upon the work of several men, a new mathematical model was developed to describe the structure of the atom.Louis de Broglie ( )Werner Heisenberg ( )Erwin Schrodinger ( )
27Previous models were physical models based upon the motion of objects Previous models were physical models based upon the motion of objects. This model is primarily a mathematical model and had never been proven....
32Atomic Number 6 6 15 15 79 79 In a neutral atom: Atomic number of an element is the number of protons in the nucleus of each atom of that element.ElementAtomic ## of protons# of electronsCarbon6Phosphorus15Gold796615157979In a neutral atom:# electrons = # protons
33The Atomic ScaleMost 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 the electron cloud.
34Atomic Particles Particle Charge Mass (g) Location Electron e- -1 9.109 x 10-28(1/1840 amu)Electron cloudProtonp++11.673 x 10-24(1 amu)NucleusNeutronn1.675 x 10-24
35The Atomic ScaleMost 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 space“q” is a particle called a “quark”
36About 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 togetherby “gluons”
37Reading the Periodic Table 3Li6.941LithiumAtomic Number# p+# e- (in a neutral atom)Element SymbolElement NameAtomic Mass# p+ + # n0# n0 = Atomic Mass - #p+
38Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons
39Mass Number Mass # = p+ + n0 Mass number is the number of protons and neutrons in the nucleus of an isotope.Mass # = p+ + n0
40C C Cl Symbols of Isotopes 6 Mass # 12 Carbon-12 Atomic Number 14 Chlorine-35Mass #Cl3517
41Mass Number Mass # = p+ + n0 O As P Isotope p+ n0 e- Mass # Oxygen - 10-3342-311518O1888818As7533Arsenic753375P3115Phosphorus161531
42Isotopes…Again (must be on the test) Isotopes 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
43Composition of the nucleus Atomic MassesAtomic mass is the weighted average of all the naturally occuring isotopes of that element.Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.IsotopeSymbolComposition of the nucleus% in natureCarbon-1212C6 protons6 neutrons98.89%Carbon-1313C7 neutrons1.11%Carbon-1414C8 neutrons<0.01%Carbon =
44Find the missing percentage. Silver has two naturally occurring isotopes:Isotope nameIsotope mass (amu)percentageSilver-10751.86Silver-109remainderFind the missing percentage.Find the average atomic mass of an atom of silver.
45Isotope name Isotope mass (amu) Relative Abundance Silicon has three naturally occurring isotopes:Isotope nameIsotope mass (amu)Relative AbundanceSilicon-2827.9892.21Silicon-2928.984.70Silicon-3029.973.09Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28, 29, or 30?Find the average atomic mass of silicon.
46Isotope name Isotope abundance Isotope mass (amu) Iron has four naturally occurring isotopes:Isotope nameIsotope abundanceIsotope mass (amu)Iron-545.90%53.94Iron-5691.72%55.93Iron-572.10%56.94Iron-580.280%57.93Estimate the average mass.Find the average atomic mass of iron.