Bires, 2010 Chapter 3: The Atom and the Mole (with nuclear) The investigation and understanding of the atom is what chemistry is all about! Topics rearranged from your text, pages We come here to be philosophers, and I hope you will always remember that whenever a result happens, especially if it be new, you should say, “What is the cause? Why does it occur?” and you will, in the course of time, find out the reason. -Michael Faraday

Bires, 2010 The Mole The “mole” represents a number of things….like a dozen. The “mole” represents a number of things….like a dozen. How many things is a mole? How many things is a mole? x … we use 6.02 x x … we use 6.02 x This is Avogadro’s number This is Avogadro’s number –named for a lawyer, Amadoe Avogadro, that studied molecular gasses as a hobby. When you have three moles of atoms, you have (3 x 6.02x10 23 =) 1.81x10 24 atoms total. When you have three moles of atoms, you have (3 x 6.02x10 23 =) 1.81x10 24 atoms total.

Bires, 2010 Recall: Parts of the atom (subatomic particles) Proton  charge = +1, mass = 1 Proton  charge = +1, mass = 1 Neutron  charge = 0, mass = 1 Neutron  charge = 0, mass = 1 Electron  charge = -1, mass = 0 Electron  charge = -1, mass = 0 In a normal, neutral, unreacted atom, the number of electrons equals the number of protons. In a normal, neutral, unreacted atom, the number of electrons equals the number of protons Ions have more or less electrons than protons They have a charge

Bires, 2010 Atomic History Greek philosopher Democritus (400BC) Greek philosopher Democritus (400BC) –coined the term atomon which means “that which cannot be divided.” John Dalton (1803) a colorblind chemist. John Dalton (1803) a colorblind chemist. –Among his interests, Dalton was very interested in a scientific explanation for his colorblindness the behavior of gasses. In his A New System of Chemical Philosophy, Dalton published five principles of matter. In his A New System of Chemical Philosophy, Dalton published five principles of matter.

Bires, 2010 Dalton’s Top Five All matter is made of indestructible and indivisible atoms. All matter is made of indestructible and indivisible atoms. –(atoms are hard, unbreakable, the smallest thing there is) Atoms of a given element have identical physical and chemical properties. Atoms of a given element have identical physical and chemical properties. –(all atoms of X will behave the same anywhere) Different atoms have different properties. Different atoms have different properties. –(X behaves differently than Y) Atoms combine in whole-number ratios to form compounds. Atoms combine in whole-number ratios to form compounds. –(two H’s and one O = Water (H 2 O) Atoms cannot be divided, created or destroyed, Atoms cannot be divided, created or destroyed, –(just rearranged in chemical reactions).

Bires, 2010 The Laws: Constant Composition Constant Composition –Ratios of atoms in a compound is constant for that compound. Conservation of Mass Conservation of Mass –Mass is not created or destroyed in a chemical reaction. Multiple Proportions: Multiple Proportions: –Since atoms bond in small, whole number ratios to form compounds, the ratio of their mass ratios are small whole numbers. Oxygen-carbon mass ratio = 1.33 Oxygen-carbon mass ratio = 2.66 x 2 hydrogen-oxygen atomic ratio = 2:1

Bires, 2010 Conservation of Mass

Bires, 2010 Multiple Proportions

Bires, 2010 The Cathode Ray Tube The cathode ray tube The cathode ray tube –A new invention suggested the presence of charges – areas of positive and negative…charge. –This suggested that atoms must be divisible, and Dalton’s theory had to be modified. J. J. Thomson (1897) J. J. Thomson (1897) –English Physicist proposed that the atom is a sphere of positive charge with small areas of negative charge. –This theory become known as the “plum pudding” model after an English “dessert” of purple bread and raisins. Electrostatics

Bires, 2010 Millikan’s Oil Thompson used electrostatics experiments to determine the electron’s charge-to-mass ratio. Thompson used electrostatics experiments to determine the electron’s charge-to-mass ratio. Robert Millikan’s (1909) Robert Millikan’s (1909) –oil-drop experiment allowed the charge of a single electron to be determined: 1.60 x C. Scientists calculated the mass of an electron to be 1/2000 of the mass of a proton! Scientists calculated the mass of an electron to be 1/2000 of the mass of a proton!

Bires, 2010 Ernest Rutherford Ernest Rutherford (1910) Ernest Rutherford (1910) –New Zealander Physicist, while studying radioactive elements, found that radioactive alpha particles deflected when fired at a very thin gold foil. The Gold Foil Experiment The Gold Foil Experiment –the atom was not a hard sphere but –was mostly space, with a small concentration of positively-charged mass (the nucleus). Link to experiment Link to experiment Link to experiment Link to experiment

Bires, 2010 The Bohr Model Niels Bohr Niels Bohr –A Danish physicist (and student of Rutherford) rebuilt the model of the atom placing the electrons in energy levels. Bohr was one of the founders of quantum chemistry: Bohr was one of the founders of quantum chemistry: –energy can be taken in and given off in small packets or quanta of specific size. –When a specific amount of energy was added to an atom, an electron could jump into a higher energy level. No more…no less!

Bires, 2010 Adding the Neutrons James Chadwick (1932) James Chadwick (1932) –British physicist, proved there was too much mass in the nucleus –Suggested the existence of massive, neutral particles in the nucleus. (neutrons)

Bires, 2010 The Modern Model (not to scale) Chadwick’s neutrons Rutherford’s space and nucleus Dalton’s atom Bohr’s energy levels Thompson’s electrons

Bires, 2010Elements 112 known elements 112 known elements –92 of which are naturally occurring. –93 through 112: transuranium. Each has an atomic symbol. Each has an atomic symbol. Atomic number Atomic number –is number of protons Atomic mass Atomic mass –is the total mass of the protons plus the neutrons. O OXYGEN Notice that the atomic mass is not a round number, even though protons and neutrons each have a mass of 1. This is due to natural abundance.

Bires, 2010 Natural Abundance - Isotopes Isotopes: Isotopes: –Each element may have several isotopes –Isotopes differ in the number of neutrons. Example: Example: –the element carbon has 6 protons, but it could have 5, 6, 7, or 8 neutrons, to form Carbon-11, Carbon- 12, Carbon-13, and Carbon-14. In nature, there is a mix of different natural isotopes. In nature, there is a mix of different natural isotopes. We use this mix to calculate average atomic mass… We use this mix to calculate average atomic mass… 11 C, 12 C, 13 C, 14 C

Bires, 2010 Calculating Average Atomic Mass Sum of the products = average atomic mass Sum of the products = average atomic mass Example: Example: –The isotopes of element “Bob” are found below: –Bob-18.0, 25.0% –Bob-19.0, 60.0% –Bob-20.0, 15.0% –What is the average atomic mass of naturally occurring Bob? 1 amu = 1.66x kg x x x   

Bires, 2010 Review … Isotopes Isotopes –atoms of the same _______ –different number of _______ Ions Ions –atoms of the same _______ –different number of _______ Allotropes Allotropes –forms of the same _______ –bonded in different _______ Quanta / Quantum Quanta / Quantum –Packets of energy of _______ size Atomic Mass Atomic Mass –Is the _______ of all _______ found in nature.

Bires, 2010 Molar Mass Molar mass Molar mass –expressed in grams per mole (g/mol) –mass of one mole of a substance. –link between the atom and the gram. (we can measure) The average atomic mass of carbon is What is the mass of a mole of carbon atoms? The average atomic mass of carbon is What is the mass of a mole of carbon atoms? What is the molar mass of Copper, Cu? What is the molar mass of Copper, Cu? What is the molar mass of Nitrogen, N 2 ? What is the molar mass of Nitrogen, N 2 ? No more AMU: AMU  Molar mass

Bires, 2010 Molar Mass Practice Determine the Molar Mass of the following elements and compounds: Determine the Molar Mass of the following elements and compounds: Ca Ca Cl 2 Cl 2 CaCl 2 CaCl 2 H 2 O H 2 O Ba(OH) 2 Ba(OH) 2 FeSO 4 FeSO 4 Al 2 (SO 4 ) 3 Al 2 (SO 4 ) 3

Bires, 2010 Mole-to-Mass Conversions Sodium has an atomic mass of 23 g/mol. How many moles do you have in 115 grams? Sodium has an atomic mass of 23 g/mol. How many moles do you have in 115 grams? How many grams are equal to 3.5 moles of CaCl 2 ? How many grams are equal to 3.5 moles of CaCl 2 ? What is the mass of 0.46 moles of SiO 2 ? What is the mass of 0.46 moles of SiO 2 ? Use a T-chart!

Bires, 2010 Mole-Mass Conversion Practice Complete the following mole-to-mass conversions: Complete the following mole-to-mass conversions: Mass in grams of 2.25 moles of iron, Fe? Mass in grams of 2.25 moles of iron, Fe? 126 grams Fe 126 grams Fe Mass in grams of moles of potassium, K? Mass in grams of moles of potassium, K? 14.7 grams K 14.7 grams K Number of moles in 5.00 grams of calcium, Ca? Number of moles in 5.00 grams of calcium, Ca? moles Ca moles Ca Number of moles in 3.60x grams of gold, Au? Number of moles in 3.60x grams of gold, Au? 1.83x mol Au 1.83x mol Au Use your periodic table to find molar mass

Bires, 2010 Mole-Atoms Conversions Mole = 6.02x10 23 things, how many atoms are in: Mole = 6.02x10 23 things, how many atoms are in: 3.0 moles of silver, Ag? 3.0 moles of silver, Ag? moles of copper, Cu? moles of copper, Cu? How many moles do you have in: How many moles do you have in: 2.4x10 24 atoms of helium, He? 2.4x10 24 atoms of helium, He? 3.0x10 23 atoms of lithium, Li? 3.0x10 23 atoms of lithium, Li? How many moles do you have in 222 grams of copper? How many moles do you have in 222 grams of copper? How many atoms in grams of copper? How many atoms in grams of copper? End of Chapter 3 Phew!

Bires, 2010 Isotopes – Nuclides - Radioactivity Nuclides Nuclides –the nucleus of an isotope Place the mass above the charge as seen here. Place the mass above the charge as seen here. Nuclides undergo decay: Nuclides undergo decay: –transformation into different nuclides –Balanced nuclear reactions –“Radioactive” –Half Life: time to decay ½ (mass) of a sample Images from ChemZoneChemZone mass charge

Bires, 2010 Atomic Stability Recall: Recall: –Nuclei = positively-charged protons –and neutral neutrons. p/n called “nucleons” Protons repel each other. Protons repel each other. –Without neutrons, protons would cause the nucleus to fly apart. Neutrons act like “nuclear glue”, interacting with a force we call the strong nuclear force. Neutrons act like “nuclear glue”, interacting with a force we call the strong nuclear force. When the ratio of neutrons to protons is too high or too low, the nucleus becomes unstable and will release particles to become stable. When the ratio of neutrons to protons is too high or too low, the nucleus becomes unstable and will release particles to become stable.

Bires, 2010 Unstable Nuclei – General Rules Nearly all elements have a radioactive isotope. Nearly all elements have a radioactive isotope. All isotopes of atoms above atomic number 83 are radioactive. ( Why this is? ) All isotopes of atoms above atomic number 83 are radioactive. ( Why this is? ) Very large nucleus = nuclear glue is too weak to hold all the particles together. (Instability!) Very large nucleus = nuclear glue is too weak to hold all the particles together. (Instability!) Small nuclides = 1:1 n:p ratio = stability Small nuclides = 1:1 n:p ratio = stability Large nuclides = 1.5:1 n:p ratio = stability. Large nuclides = 1.5:1 n:p ratio = stability. What if the ratio of protons-neutrons is way off? What if the ratio of protons-neutrons is way off?

Bires, 2010 Band of Stability A graph of neutrons-protons shows the stable isotopes, (acceptable n:p ratios) A graph of neutrons-protons shows the stable isotopes, (acceptable n:p ratios) Nuclides outside this range are unstable Nuclides outside this range are unstable –They will DECAY –3 basic types of radioactivedecay unstable stable

Bires, 2010 Alpha Decay Alpha Decay Alpha Decay –a helium nucleus is released. Alpha particles: Alpha particles: –move very slowly –because of their size, can be blocked with a few pages of paper or human skin –cause ionization (damaging!) –are positively charged Images from ChemZoneChemZone mass charge Alpha Decay occurs in all elements with atomic number above 83. This is a Nuclear Equation

Bires, 2010 Beta Decay Beta Decay Beta Decay –An electron is ejected from the nucleus Beta particles Beta particles –move fast –can penetrate thick low- density materials –but can be blocked with concrete and metals –are negatively charged Images from ChemZoneChemZone Beta Decay occurs when a nucleus has a high neutron-proton ratio.

Bires, 2010 Gamma Decay Gamma Decay Gamma Decay –High energy photons (gamma rays) are given off. Gamma rays Gamma rays –given off as the “spare change” during other radioactive decays…. –extremely penetrating and powerful. Several inches of lead is required to slow these particles down to a stop. –Don’t get included in nuclear equations. Images from ChemZoneChemZone Summary of three basic particle decays No mass No charge

Bires, 2010 Other decays: Positron Decay Positron decay Positron decay –an anti-electron is given off from the nucleus –Same properties as beta decay, but… Notice the charge of +1 on the positron. Notice the charge of +1 on the positron. Images from ChemZoneChemZone Positron emission occurs when a nucleus has a low neutron-proton ratio. “Parent nuclide” “Daughter nuclide” This is a Nuclear Equation

Bires, 2010 Other decays: Neutron Emission Neutron emission Neutron emission –a high-energy neutron is given off Neutron emission: Neutron emission: –Reduces the n:p ratio –Sometimes accompanies other types of radioactive decay –Makes nuclear weapons possible –Is extremely destructive to living tissue –Is the fuel source reaction for nuclear power reactors

Bires, 2010 Nuclear Equations Practice Complete the following nuclear equations: Complete the following nuclear equations: Sodium-24 undergoes alpha decay (help on click) Sodium-24 undergoes alpha decay (help on click) Iodine-131 undergoes beta decay Iodine-131 undergoes beta decay Tungsten-190 undergoes alpha decay Tungsten-190 undergoes alpha decay Uranium-238 undergoes alpha decay, then two beta decays (3 steps) Uranium-238 undergoes alpha decay, then two beta decays (3 steps)

Bires, 2010 Nuclear Fission Nuclear fission: Nuclear fission: –splitting of large, unstable atoms –releases large amounts of energy Critical mass (or critical density) Critical mass (or critical density) –amount of fissionable fuel needed before reaction will begin. Uncontrolled, nuclear fission proceeds to completion with great speed. Uncontrolled, nuclear fission proceeds to completion with great speed. Nuclear Weapons: Nuclear Weapons: –two half-spheres of fissionable material are compressed together with conventional explosives, creating the critical mass. In order to harness nuclear fission to create useable electricity, we slow down the process with control rods… In order to harness nuclear fission to create useable electricity, we slow down the process with control rods… Once fission begins, it is difficult to stop.

Bires, 2010 Fission Reactor Power Plants In the reactor: In the reactor: –heat is generated when high-energy neutrons slow down in collisions with the moderator. Pressurized Steam Reactor: Steam is produced and drives turbines, creating electricity. Pressurized Steam Reactor: Steam is produced and drives turbines, creating electricity. Reactor fuel must be fissionable, usually weapons-grade uranium (U) or plutonium (Pu). Reactor fuel must be fissionable, usually weapons-grade uranium (U) or plutonium (Pu). Moderators become highly radioactivity (dirty). Moderators become highly radioactivity (dirty). Popular Moderators: B (s), Cd (s), Na (l), Li (l) 238 U – natural – not fissionable 235 U – enriched – fissionable

Bires, 2010 Nuclear Fusion Nuclear Fusion: Nuclear Fusion: –Joining of smaller nuclei to form larger nuclei. –Releases far more energy that nuclear fission. –Easier to control than fission. The sun’s (stars) energy comes from the fusion of hydrogen atoms into helium atoms. The sun’s (stars) energy comes from the fusion of hydrogen atoms into helium atoms. The H-bomb is a fusion weapon. The H-bomb is a fusion weapon. Fusion: Fusion: –The power supply of the future? –Why don’t we use it now?

Bires, 2010 E=mc 2 Einstein: Energy and mass are interchangeable. Einstein: Energy and mass are interchangeable. –E = Energy –m = mass –c = speed of light ( 3 x 10 8 m/s ) –Very small amounts of mass create large amounts of energy! We use the formula ΔE= Δmc 2 to build new, artificial elements in supercolliders (particle accelerators.) We use the formula ΔE= Δmc 2 to build new, artificial elements in supercolliders (particle accelerators.) Fermilab cyclotron, Argonne National Laboratory, Chicago

Bires, 2010 CCSD Syllabus Objectives 4.3: Conservation of Mass 4.3: Conservation of Mass 4.4: Symbols and Formulas 4.4: Symbols and Formulas 5.1: History of the Atom 5.1: History of the Atom 5.2: Atomic Structure 5.2: Atomic Structure 5.3: Atomic Number, Mass 5.3: Atomic Number, Mass 5.4: Molar Mass 5.4: Molar Mass 5.5: Quantum Model of the Atom 5.5: Quantum Model of the Atom 20.1: Alpha, Beta, Gamma Decay 20.1: Alpha, Beta, Gamma Decay 20.2: Half Life 20.2: Half Life 20.3: Nuclear Equations 20.3: Nuclear Equations 20.4: Fission and Fusion 20.4: Fission and Fusion 20.5: Nuclear Material 20.5: Nuclear Material