The Atom: SECTION 4.2

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The Atom: SECTION 4.2. Defining the Atom. Elements are made of ATOMS ATOM: The smallest particle of an element that retains the properties of the element. BELIEVING IN THE ATOM. For a long time, we had to INFER atoms existence through scientific experiments. - PowerPoint PPT Presentation

Transcript of The Atom: SECTION 4.2

  • The Atom: SECTION 4.2

  • Defining the AtomElements are made of ATOMSATOM: The smallest particle of an element that retains the properties of the element

  • BELIEVING IN THE ATOMFor a long time, we had to INFER atoms existence through scientific experiments.Now, we have the technology to SEE atomsSCANNING TUNNEL MICROSCOPE

  • How small is an atom?Now I'd like you to picture yourself on the beach. Reach down and pick up a handful of sand. How many grains of sand do you see? Could you even begin to count the number of grains of sand you are holding in that one handful? Imagine picking out one single grain of sand. There are possibly more atoms within that single grain of sand than there are grains of sand on the entire beach.About 5 million atoms could fit in the period at the end of a sentence.

  • Daltons FlawsTwo of his points were found to be FALSEAtoms can be broken into subatomic particlesSome atoms of the same element can have different properties

  • ElectronsAtoms are made up of smaller particlesElectron: The negatively charged particles of an atomAll atoms have e- (electrons)

  • Questions

  • Cathode Ray TubeScientists pass electricity through a glass tube with NO AIR inside (vacuum)PURPOSE: to study the behavior of electricity without matter.Helped DISCOVER THE ELECTRON

  • Cathode Ray TubeThe glass tube is hooked up to a battery.Negative plate (cathode) hooks up to - part of batteryPositive plate (anode) hooks to + side of battery

  • Cathode Ray+-Vacuum tubeMetal Disks

  • Sir William CrookesSaw a RAY going through tubeHe observed that it originated from the cathode.He called it the CATHODE RAY

  • Passing an electric current makes a beam move from the negative to the positive end-+

  • Adding charged plates outside the tube caused the beam to be deflectedWhy was the beam deflected towards the positive plate?-+

  • WHAT IS THE BEAM??The CATHODE RAY consists of CHARGED particlesBecause they came FROM the negative end (cathode) TO the positive end (anode) MUST BE Because they were deflected TOWARDS + plate, MUST BE (opposites attract)Negatively charged particles = ELECTRONS

  • Oil-drop experimentMillikanSprayed oil droplets that fell btw CHARGED plates (- plate is on the bottom)Different amounts of charge changed the droplets rate of fallMore charge = falls more slowly (repelled by - plate)

  • Millikans ExperimentDetermined the charge of an electron is -1

  • Mass of an ElectronHe used the charge and the charge to mass ratio to mathematically figure out the mass of an electron.Mass of electron = 9.1 x 10-28 kg 1840 times SMALLER than the smallest atom

  • Structure of the Atom (a guess)Atoms are NEUTRAL so if they have electrons, they must also have + chargeHow are electrons arranged in an atom??

  • Thomsons Model

    - Thought the atom was arranged like PLUM PUDDING- Electrons spread evenly throughout an atoms positive interior

  • QUESTIONSDraw the Plum Pudding modelWhy is this model of the atom called plum pudding?

  • RutherfordErnest Rutherford studied alpha particles (+ charged)He shot a narrow beam of alpha particles at a thin sheet of gold foil

  • RutherfordBecause + and charge were thought to be evenly distributed, + particles should go straight through foil (not attracted or repelled)

  • Rutherfords Experiment

  • Gold Foil Atoms Alpha Particle Beam

  • WHAT HE SAWRutherfords experimentMost particles passed straight through the foil as if the foil was not there (as expected)A very small fraction of alpha particles were deflected in ALL directions some even completely reversed direction

  • An animation

  • What this tells RutherfordAlpha (+) particles should not get deflected dramatically by even distribution of chargeThere must be a dense, massive concentration of positive charge in the center

  • What was happening on an atomic level Positive Center Alpha beam deflected because positive charges repel

  • Rutherfords ConclusionMost of atom is empty spaceThere is a concentrated positive center: NUCLEUSNucleus is DENSE and has most of atoms MASS (electrons are MUCH lighter than nucleus)

  • Rutherfords modelElectrons stay with the atom because they are attracted to the positive centerElectrons orbit the nucleus similar to planets around the sun

  • Rutherford eventually concluded

    - Nucleus- Contains POSIVITELY charged particles called PROTONS and NEUTRALLY charged particles called NEUTRONS

  • NeutronsNeutrons are the other particle in the nucleusThey have no chargeIts mass is nearly equal to that of the proton

  • HomeworkPg 114 #7, 8, 9 and 10

  • 4.3 How Atoms Differ

  • Atomic Number- Atomic Number = # protons in an atom- # of protons determines kind of atom (atoms of an element always have the SAME number of protons)The atomic number = the number of electrons in a neutral atomWritten just above the atomic symbol on the periodic table

  • Complete the Table:

    ElementAtomic ## Protons# Electrons6NPb319Br

  • ElementAtomic ## Protons# ElectronsC666N777Pb828282Li333K191919Br353535

  • IsotopesAll atoms in an element have the same number of protonsThe number of neutrons varyAtoms of an element with different numbers of neutrons = ISOTOPES

  • IsotopesElements in nature = a mix of isotopesThe abundances of different isotopes are constantEX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons

  • Mass of atomsOne atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.One amu is nearly, but not exactly, equal to one proton or one neutron.

  • Mass of subatomic particles

  • Mass Number# PROTONS + # NEUTRONS = MASS NUMBEREXAMPLE: What is the mass number for manganese if there are 30 neutrons in the nucleus?

  • Element SymbolsTop is mass numberBottom is atomic number


  • SymbolsEXAMPLE 1: Find the number of protonsnumber of neutronsnumber of electronsAtomic numberMass NumberF19 9

  • Answer9 protons10 neutrons9 electronsAtomic # = 9Mass # = 19What would the mass number be of a fluorine isotope with two additional neutrons?

  • SymbolsEXAMPLE:Find the number of protonsnumber of neutronsnumber of electronsAtomic numberMass NumberBr80 35

  • AnswerNumber of protons: 35Number of neutrons: 45Number of electrons: 35Atomic number: 35Mass Number: 80

  • SymbolsIf an element has an atomic number of 34, a mass number of 78, findnumber of protonsnumber of neutronsnumber of electronsAnd WRITE the Complete symbol

  • Answer34 protons34 electrons44 neutronsSymbol:


  • Symbolsif an element has 91 protons and 140 neutrons what is the Atomic numberMass numbernumber of electronsComplete symbol

  • Pa91231

  • Symbolsif an element has 78 electrons and 117 neutrons what is the Atomic numberMass numbernumber of protonsComplete symbol

  • Pt78195

  • Naming IsotopesPut the mass number after the name of the elementcarbon- 12carbon -14uranium-235

  • Mass of the atomAtomic mass on the periodic table is NOT an even #The atomic mass of an element is the weighted average mass of the isotopes of that elementDepends on abundance of each isotope

  • Atomic MassHow heavy is an atom of oxygen?The MASS NUMBERS weve been using are measured in amu1 amu (atomic mass units) is ABOUT the mass of 1 proton or 1 neutron (or 1/12 of a carbon -12 atom)On the periodic table, the MASS of each element is NOT reported as a whole numberWe cant have HALF a proton or HALF a neutron in an atom so why arent these masses whole numbers??

  • Atomic MassChlorine-35 is MORE common in nature than chlorine-37so the mass of 1000 Cl atoms will be CLOSER to 35000 amuTo take the average of the two isotopes, we need to use the WEIGHTED AVERAGE:In nature, 75.53% of Cl atoms are chlorine-35, and 24.47 % are chlorine- 37(% is called % abundance)

  • Calculating the WEIGHTED AVERAGEWrite each percent as a decimalMultiply that decimal by the isotopes massFind the average by adding together Average = % as decimal x mass + % as decimal x mass

    Average Mass of a Chlorine atom: (0.7553)(35) + (0.2447)(37) = 35.4894 amu% abundance isotope 1Mass isotope 1% abundance isotope 2

    Mass isotope 2

  • Atomic Mass: EXAMPLE- Calculate the atomic mass of copper if copper has two isotopes. 69.1% have a mass of 62.93 amu and the rest have a mass of 64.93 amu

  • ANSWERMass of Cu = (.691)(62.93) + (.309)(64.93) = 63.548 amu

  • Atomic Mass: EXAMPLE 2- Magnesium has three isotopes. 78.99% magnesium-24, 10.00% magnesium-25, and the rest is magnesium-26 amu. What is the atomic mass of magnesium?

  • AnswerMass of Magnesium = (.7899 x 24) + (.1000 x 25) + (.1101 x 26) = 24.3202 amu

  • 4.4 Unstable Nuclei and Radioactive Decay

  • Radioactive decayIn the late 1890s, scientists noticed some substances spontaneously emitted radiation, a process they called radioactivity.

  • Radioactive DecayNuclear reactions can change one element into another element.

  • Radioactive decayradiation is r