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1 DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY Chemistry 140 Fall 2002TENTH EDITIONGENERAL CHEMISTRYPrinciples and Modern ApplicationsPETRUCCI HERRING MADURA BISSONNETTE2Atoms and the Atomic TheoryPHILIP DUTTONUNIVERSITY OF WINDSORDEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY
2 Atoms and the Atomic Theory Chemistry 140 Fall 2002Contents2-1Early Chemical Discoveries and the Atomic Theory2-2Electrons and Other Discoveries in Atomic Physics2-3The Nuclear Atom2-4Chemical Elements2-5Atomic Mass2-6Introduction to the Periodic Table2-7The Concept of the Mole and the Avogadro Constant2-8Using the Mole Concept in CalculationsAtoms and the Atomic TheoryImage of silicon atoms that are only 78 pm apart; image produced by using ascanning transmission electron microscope (STEM). The hypothesis that all matter ismade up of atoms has existed for more than 2000 years. It is only within the last fewdecades, however, that techniques have been developed that can render individualatoms visible.
3 2-1 Early Discoveries and the Atomic Theory Lavoisier 1774 Formulate the law of conservation of massHe heated a sealed glass vessel containing a sample of tin and some airThe mass before heating (vessel + tin + air)=after heating (vessel + tin + air)The total mass of the substance present after a chemical reaction is the same as the total mass of substance before the reaction.
4 Figure show the reaction between silver nitrate and potassium chromate to give a red solid (silver chromate)(a) Before the reaction, the beaker with a silver nitrate solution and a graduated potassium chromate solution are placed on a single pan balance displace the combine mass = g(b) After mixing, a chemical reaction occurs that forms silver chromate (red precipitate) in potassium nitrate solution. The total mass = g, remains unchanged.FIGURE 2-2Mass is conserved during a chemical reactionThe low of conversation of mass says that matter is neither created nor destroyed in a chemical reaction
6 Sample A and Its Composition Proust 1799 Law of constant compositionAll sample of the compound have the same composition- the same proportions by mass of the constituent elements.Consider the compound water made up of two atoms of hydrogen (H) for every atoms of oxygen (O)Can be presented chemical formula H20Two samples describes below have the same proportions of the two elementsExp: determine the percent by mass of hydrogenSimple divide the mass of hydrogen by the sample mass and multiply by 100%. For each sample, you will obtain the same results:11.19% HSample A and Its Compositiongg1.119 g H% H=11.193.021 g H8.881 g O% O= 88.81g O
8 Dalton’s Atomic Theory: John Dalton Describes the basis of atomic theory with three assumptionsEach element is composed of small particles called atoms. Atoms are neither created nor destroyed in chemical reactions.All atoms of a given element are identical but atoms of one element are different from those off all other elementsCompounds are formed when atoms of more than one element combine in simple numerical ratios.exp: one atom of A to two B (AB2)
9 The second oxide is richer in O In forming carbon monoxide (CO), 1.0 g of carbon combines with 1.33 g of oxygen.In forming carbon dioxide (CO2), 1.0 g of carbon combines with 2.66 g of oxygen.Figure 2-3Molecules of CO and CO2The second oxide is richer in OIt contains twice as much O as the first
10 2-2 Electrons and Other Discoveries in Atomic Physics Electricity and magnetism were used in the experiment so that led to the current theory of atomic structureCertain objects displays a properties called electric charge, which can be either positive (+) or negative (-)An object having equal number of (+) or (-) charged particles carries no net charge and is electrically neutralIf the number of (+) charge exceed the number of (-) charge , the object has a net positive chargeIf the number of (-) charge exceed the number of (+) charge , the object has a net negative charge
11 Forces between electrically charged objects (+) and (-) charges attract each other , while two (+) and two (-) charges repel each otherFIGURE 2-4Forces between electrically charged objects(a) Electrostatically charged comb. If you comb your hair the static chargedevelop on the comb and causes bits of paper to be attracted to the comb(b) Both objects on the left carry negative charge repel each otherThe objects in the center lack any electrical charge and exert no force on each otherThe object on the right carry opposite charges and attract each other
12 The Discovery of Electrons Faraday discovered cathode rays, a type of radiation emitted by a (-) terminal, or cathode (it is iron, platinum so on)The radiation crossed the evacuated tube to the (+) terminal, or anodeFIGURE 2-6Cathode ray tubeThe high voltage source of electricity creates a (-) charge on the electrode at the left (cathode) and a (+) charge on the electrode at the right (anode)Cathode rays pass from the cathode (C) to the anode (A) which is perforated to allow the passage of a narrow beam of the cathode raysThey are visible only through the green florescence that they produce on the zinc sulfide-coated screen at the end of the tube. They are in the other part s of the tube
13 Cathode rays and their properties C rays are deflected by electric and magnetic fields in the manner expected for negatively charged particles (Figure 2-7 (a) and (b))Figure 2-7 (c) determine the mass to charge ratio m/e for the C raysCathode rays subsequently become known as electronsElectron m/e = × 10-9 g coulomb-1FIGURE 2-7Cathode rays and their properties
14 Millikan’s oil-drop experiment Robert Millikan determined the electronic charge , e through a serious oil drop experimentHe showed ionized oil drops can be balanced against the pull of gravity by an electric fielde is x10-19 coulombmass of electron= ( x10-19 coulomb) x ( × 10-9 g coulomb-1 ) = x gFigure 2-8Millikan’s oil-drop experiment
15 Plum-pudding Model Proposed by Thomson Explains how the electron particles were incorporated into atoms.He thought that the positive charge necessary to counter balance the negative charges of electrons in a neutral atom was the form of a nebulous cloud.He suggest, electrons floated in a diffuse cloud of positive chargeA helium atom would have a +2 cloud of (+) charge and two electrons (-2)If helium atom loses one electron, it becomes charged and is called an ion referred to He+ has a net charge of 1+If the helium atom loses both electron the He2+ ion forms
16 X-Rays and Radioactivity X-ray is form of high energy electromagnetic radiationRadioactivity is the spontaneous emission of radiation from a substanceTwo types of radiation form from radioactive material were identified by Ernest RutherfordAlpha (a): a-particles carry two fundamental units of positive charge and the same mass as helium atoms. This particle are identical to He2+ionsBeta (b): b-particles are negatively charged and have the same properties as electronsGamma (g) rays: is not effected by electric or magnetic field. It is not made of particles. It is electromagnetic radiation of extremely high penetrating power.
17 2-3 The Nuclear Atom The scattering of a- particles by metal foil Rutherford used the a-particles to study inner structure of the atomsThe telescope travels in a circular track around at evacuated chamber containing the metal foil.Most a-particles pass thought the metal foil undeflected , but some are deflected through large anglesFigure 2-11The scattering of a- particles by metal foilThe nuclear atom have these features belowMost of mass and all of positive charge of an atom are centered in a very small region called nucleus. The remainder of the atom is mostly empty spaceThe magnitude of the positive charge is different for the different atoms and is approximately one-half the atomic weight of the elementThere are as many electrons outside the nucleus as there are unit of positive charge on the nucleus. The atom as a whole is electrically neutral.
18 The nuclear atom – illustrated by the helium atom Properties of Protons, neutrons and ElectronsProtons: positively charged fundamental particles of the matter in the nuclei of atomsNeutrons: penetrating radiation consisted of beam of neutral particlesThe number of protons in a given atom is called the atomic number, or the proton number, ZThe number of electrons in the atom is equal to Z because the atom is electrically neutralThe total number of proton and neutrons in an atom is called the mass number, AThe number of neutron is A-Z and electrically neutral.Figure 2-13The nuclear atom – illustrated by the helium atom
19 To represent a particular atom we use symbolism Chemistry 140 Fall 20022-4 Chemical ElementsEach element has a name and distinctive symbolExp: carbon:C, oxygen:O, neon:Ne, iron:FeTo represent a particular atom we use symbolismSymbol of elementA= mass number Z = atomic numberOften do not specify Z when writing. For example 14C, C specifies Z = 12.Special names for some isotopes. For example hydrogen, deuterium, tritium.Has 13 protons and 14 neutrons in its nucleus and 13 electron outside the nucleus (recall that an atom has the same number of electrons as protons)Al1327
20 Chemistry 140 Fall 2002Isotopesatoms that have the same atomic number (Z) but different masss number (A) are called isotopes.Exp: all neon atoms have 10 protons in their nuclei, and most have 10 neutron as well. A very few neon atoms have 11 neutrons and some have 12Ne1020Ne1021Ne1022IonsWhen atoms lose or gain electrons the species formed are called ions and carry net charges.Removing electrons result in positively charged ionThe number of proton does not change when an atom becomes an ion.Exp:Ne1020+10 protons 10 neutrons and 9 electronsOften do not specify Z when writing. For example 14C, C specifies Z = 12.Special names for some isotopes. For example hydrogen, deuterium, tritium.Ne10222+10 protons 12 neutrons and 8 electronsO8162-8 protons 8 neutrons and 10 electrons
22 A mass spectrometer and a mass spectrum Isotopic massesUsed when original mass of an atoms can not be determinedThat must be done by experimentOne type of atom has been chosen and assigned a specific mass. This standard is an atom of the isotope carbon-12Next the masses of the other atoms relative to carbon -12 are determined with a mass spectrometerFigure 2-14A mass spectrometer and a mass spectrum
24 2-5 Atomic MassThe average of the isotopic masses, weighted according to the naturally occurring abundance of the isotopes of the elementsWeighted AverageAtomic Mass of an ElementEquation (2.3)fractional abundance of isotope 1atomic mass of isotope 1fractional abundance of isotope 2atomic mass of isotope 2x+=x+ ……Aave=x1xA1+x2xA2+ ……xnxAnwhere x1 + x2+ …..+ xn = 1.0
27 2-6 Introduction to The Periodic Table The classification system we need known as the periodic table of the elementsRead atomic massesRead the ions formed by main group elementsRead the electron configurationLearn trends in physical and chemical propertiesWe will discuss these in detail in Chapter 9.
28 The Periodic tableNoble GasesAlkali MetalsMain GroupAlkaline EarthsHalogensTransition MetalsLanthanides and Actinides
30 2-7 The Concept of the Mole and the Avogadro Constant A mole: is the amount of the substance that contains the same number of elementary entities (atoms, molecules and so on)Avogadro constant or Avogadro number, NA: The amount of elementary entities in a moleNA = x 1023 mol-1Exp:1 mol 12C = x C atoms = 12 g1 mol 16O = x O atoms = g (and so on)Molar mass, M: the mass of one mole of substance , from a table of atomic massesExp: the molar mass of lithium is g/mol Li