Early Atomic Theorists John Dalton (1766-1844) Democritus (460-370 BC) All matter is composed of atoms that are indestructible and indivisible No Research Research

Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms of any one element are different from those of any other element. Atoms cannot be created, divided into smaller particles or destroyed. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

The Atom The smallest particle of an element that retains the property of that element Relative Size World Population 6000000000 Atoms in a penny 29000000000000000000000

Discovering the Electron Cathode (-) Anode (+) Experiments with electricity, using cathode ray tubes led to the discovery cathode rays. Cathode rays were a stream of charged particles The particles carried a negative charge

JJ Thompson Proposed the “Plum Pudding” model of the atom Determined the Charge to Mass ratio of cathode particles (~1/1840 the mass of a hydrogen atom) and discovered the electron Disproved Daltons theory that atoms were indivisible because he determined that the mass of a cathode particle was far less than that of a hydrogen atom. + + + + + - - - - Proposed the “Plum Pudding” model of the atom - - - -

Ernest Rutherford Discovered the nucleus of the atom with the “Gold Foil” experiment

Nuclear Model of the Atom Most of the atom consist of electrons moving rapidly through empty space. Electrons are held in place in the atom by their attraction to a positively charged nucleus Does this picture accurately represent Rutherford’s Nuclear Model?

James Chadwick Discovered the Neutron Rutherford model, consisting of electrons and protons could not account for the total mass of the atom, which led to the discovery of the neutron.

Properties of Subatomic Particles Symbol Location Relative Electrical Charge Relative Mass Actual Mass (g) Electron e- Space surrounding the nucleus 1- 1_ 1840 9.11 x 10-28 Proton p+ Nucleus 1+ 1 1.637 x 10-24 Neutron n0 1.675 x 10-24

Review C = Mass Number 6 = # protons Atomic Number = # electrons 12.011 Atomic Number = Mass Number = protons + neutrons The weighted average mass of all the isotopes of an element Change in the number of protons results in a new atom Change in the number of electrons results in an ion (+ or – charge) Change in the number of neutrons results in an isotope

Isotopes C-14 Nuclear or Nuclide notation Isotope Notation Atoms of the same element that contain the same number of protons and different numbers of neutrons. Nuclear or Nuclide notation (used in nuclear equations) Isotope Notation C-14 Mass Number Mass Number Atomic Mass

Atomic Mass Problems Boron has two naturally occurring isotopes: boron-10 (19.8%, 10.013 amu) and boron 11 (80.2%, 11.009 amu). What is the atomic mass of boron? Find the Abundance x Mass for each isotope and then add products together. 0.198 x 10.013 = 1.98 0.802 x 11.009 = 8.83 1.000 10.81 amu

Unstable Nuclei and Radioactive Decay Nuclear Reactions involve a change in the nucleus of an atom Some substances spontaneously emit radiation in a process called radioactivity due to nuclear instability (stability is determined by neutron to proton ratios) Rays and particles emitted by radioactive materials are called radiation Unstable nuclei decay until they form stable non-radioactive nuclei

Types of Radiation Alpha Beta Gamma < <

Pre-class Activity 11/11/08 What type of nuclear decay has neptunium-237 undergone in the following reaction? Alpha, a, or

Pre-Class Activity 11/12/08 Which subatomic particles are involved in chemical reactions? Which subatomic particles are involved in nuclear reaction? Electrons Protons, Neutrons and Electrons

Characteristics of Chemical and Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken and formed Occur when nuclei emit particles and/or rays Atoms remain unchanged, although they may be rearranged Atoms are converted into atoms of another element Only Valence (outermost) electrons are involved May involve protons, neutrons, and electrons. Associated with small energy changes Associated with large energy changes Reaction rates are influenced by temperature, pressure, concentration, and catalysts. Reaction rates are not influenced by temperature, pressure, concentration, and catalysts.

Relative Strength and Mass of Radioactive Particles 6.64 x 10-24 9.11 x 10-28 Increasing Mass Alpha Beta Gamma Increasing Strength (Blocked By) Not completely blocked by lead or cement Paper Metal Foil

Nuclear Stability (Electrostatic Force vs. Nuclear Force) Electrostatic force arises from the interaction between two protons (repulsive force) Nuclear force arises between protons and neutrons due to their close proximity to one another

Neutron to Proton Ration and the Band of Stability 1:1 1.5:1 As the atomic number increases, more and more neutrons are needed to create a strong nuclear force to oppose and increasing electrostatic force

Radioactive Decay and Stability Alpha Decay Reduces the number of neutrons and the number of protons in the nucleus. Alpha Decay often occurs in elements with an atomic number of 83 or higher. 1.5:1 A 1:1 B In what region of the graph would this type of decay be most effective? C

Radioactive Decay and Stability 1:1 1.5:1 Beta Decay occurs in atoms that has too many neutrons relative to its number of protons A B In what region of the graph would this type of decay be most effective? A

Radioactive Decay and Stability 1:1 1.5:1 Positron emission occurs in atoms where the number of protons is high relative to its number of neutrons A B In what region of the graph would this type of decay be most effective? B

Radioactive Decay and Stability 1:1 1.5:1 Electron Capture occurs in atoms where the number of protons is high relative to its number of neutrons A B In what region of the graph would this type of decay be most effective? B

Writing Nuclear Equations http://www.sciencegeek.net/Chemistry/taters/Unit1NuclearEquations.htm Band of Stability Practice Questions http://www.algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml

Pre-Class Activity Complete the following nuclear equation, state the type of decay and explain why this nuclide decays in this way. Electron Capture, the neutron to proton ration of Pm-142 falls below the band of stability Chapter 25 Homework Quiz – Friday 11/20 Chapter 25 Test – Tuesday 11/24

Transmutation All nuclear reactions are: transmutation reactions Conversion of an element into an atom of another element All nuclear reactions are: transmutation reactions Some transmutation reactions are induced All transuranium elements (atomic #93 and greater) have been produced through induced transmutation.

Writing Induced Transmutation Reactions Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction. Write the balanced nuclear equation for the alpha particle bombardment of Pu-239. One of the reaction products is a neutron.

Radioactive Decay Radioactive decay rates are measured in half-lives (amount of time it takes for half of a sample of radioactive nuclei to decay) Equation Possible Variables Initial amount of isotope Final amount of isotope Time elapsed Number of half-lives Half-life

Sample Problem A sample of radioactive iridium has a half life of 12 years. In 60 years, how much iridium would remain from a 50g sample? Initial= Final= Time Elapsed= # of half lives= Value of 1 half life= 50g ? 60 years 60/12 = 5 12 years

Nuclear Reactions and Energy Nuclear Fission Atoms with a mass number greater than 60 tend to undergo nuclear fission in which an atom “splits” apart. Nuclear Fusion Atoms with a mass number less than 60 tend to undergo nuclear fusion in which two lighter atoms fuse together.

Binding Energy The amount of energy required to break one mole of nuclei into individual nucleons

Reactants and Products Fission- 92235U fuel used in a chain reaction Limited resource critical mass- minimum mass to sustain chain reaction Risk of runaway chain reaction Produces radioactive waste products Disposal concerns Reaction: Fusion- 12H and 13H used as fuel extracted from sea water not a chain reaction No risk of runaway reaction Nonradioactive waste: helium Problem: needs temp of 200 Million K Reaction:

Nuclear Power- generated by a controlled fission chain reaction Moderators- slow neutrons down so they DO hit uranium fuel rods Made of water, beryllium, or graphite Intended to allow neutrons to be absorbed by uranium Control rods- absorb neutrons to slow the chain reaction Made of cadmium Inserted or withdrawn to keep temp of reaction steady

Cooling and Shielding Water- acts as a coolant and transfers heat between reactor and turbines that produce electricity Steel & concrete- surround core and protect personnel by absorbing radiation

Other Uses of Radioactivity Tracers Radioactive isotopes used to track pathways Chemistry/biology- pathways of reactions Industry and environment- path of groundwater, durability of containers Medicine- diagnose malfunctions