CHAPTER 4/25 Atomic Structure Nuclear Chemistry

A Long, Long Time Ago… Greek Philosophers 4 elements are Earth, Water, Fire, and Air Aristotle - first recorded atomic thoughts Matter is continuous (no atoms) Democritus (400 B.C.) - first recorded atomic theory Atoms are smallest part of matter, each type of matter has different atoms

A Long Time Ago… Antoine Lavoisier (1782) Used experiments in closed containers to develop the Law of Conservation of Matter Joseph Proust (1799) Analyzed water to develop the Law of Definite Proportions John Dalton (1803) Compiled past research to develop the first useful atomic theory

John Dalton Dalton’s theory had 4 major tenets 1. All matter is composed of atoms 2. Atoms are indivisible 3. Atoms of 1 element are alike, but different from those of other elements 4. Atoms combine in small, whole number ratios to form compounds

A Little While Ago… J.J. Thomson (1897) discovers the electron using a cathode ray tube Proposes the “Plum Pudding” Model of the atom

Cathode Ray Tube Experiment

The Nucleus Exists! Ernest Rutherford (1911) uses the gold foil experiment to “discover” the nucleus

The Gold Foil Experiment

Results of Gold Foil Experiment… Over 98% of the particles went straight through About 2% of the particles went through but were deflected by large angles About 0.01% of the particles bounced off the gold foil About that 0.01% of particles….. …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." E. Rutheford

Rutherford’s Conclusion

Rutherford's Nuclear Model 1. The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume of the atom 2. The nucleus is essentially the entire mass of the atom 3. The nucleus is positively charged the amount of positive charge of the nucleus balances the negative charge of the electrons 4. The electrons move around in the empty space of the atom surrounding the nucleus Coworker James Chadwick later adds neutrons

Properties of Subatomic Particles ParticleSymbolLocation Relative Electrical Charge Relative Mass Actual Mass (g) Electrone-e- Space surrounding the nucleus 1- 1_ x Proton p+p+ Nucleus x Neutron n0n0 Nucleus x

Understanding Periodic Blocks Atomic Number # of protons # of electrons Element Symbol Atomic Mass The weighted average atomic mass of isotopes of an atom Mass Number #protons + # neutrons

Can Neutrons Exist??? No, is an average mass of all natural K atoms All K atoms MUST have 19 protons Some have 20 neutrons, some 19, some 21… Average is neutrons Individual isotopes are identified by the number of neutrons

Identifying Protons, Neutrons, and Electrons Chlorine-35 (element-mass number) # of protons = 17 # electrons = # protons # neutrons = mass number – protons = 18 Why no electrons in atomic mass? Electron has mass 1/1837 of Proton and Neutron

Isotopes Nuclear Notation or Isotope Notation # protons = ______ # neutrons = ______ # electrons = ______ Problems that can arise.. Charges and Ions – Only changes the number of electrons Finding the mass number – use symbol, p + and n o or periodic table in that order!

Atomic Mass Units Carbon-12 6 protons 6 neutrons atomic mass unit is equal to one-twelfth of the mass of the nucleus of a carbon-12 atom

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

Chemical vs. Nuclear Change Chemical Change- produces new kinds of matter with new properties Involves breaking and forming BONDS Accomplished by rearrangement of ELECTRONS Nuclear Change- produces a new nucleus that contains less energy Involves emission or capture of nuclear particles Accomplished by changing PROTONS and NEUTRONS in nucleus

HighMedLow Actual Hazard Low-MedMedHigh Biological Hazard LowMedHighEnergy Hard (2 cm lead) Medium (aluminum foil) Easy(skin/clothes)Shielding 00γ00γ00γ00γ 0 -1 e 4 2 He Symbol GAMMA (γ) BETA (β) ALPHA (α)

Other Radiation Ionizing Radiation- has significant energy to change atoms and molecules into ions Types: alpha, beta, gamma, x-rays Effects to living organisms: changes in, DNA (cell death/cancer) Nonionizing radiation- does not have significant energy to ionize atoms or molecules (types: microwaves, visible light, radiowaves)

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 As the atomic number increases, more and more neutrons are needed to create a strong nuclear force to oppose and increasing electrostatic force 1:1 1.5:1

Nuclear Reactions

Nuclear Decay

Radioactive Decay Half-lives Half-life is the time taken for half of the atoms of a radio-active substance to decay. Number of half-lives TE/t 1/2 5 variables to consider 1.Initial quantity 2.Final quantity 3.# of half-lives 4.Value of a half life 5.Time Elapsed

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 12 years 60 years ? 60/12 = 5

Half-life Half-lives can range from a millionth of a second to millions of years

Radioactive Dating Uses carbon-14 to tell age of fossils C-14 is present constantly in atmosphere 15.3 decays/min in living organism decays/min decreases by ½ every 5370 years an organism is dead. Only useful to 60,000 yrs ago

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

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.

Nuclear Reactions for Energy Fission- nucleus broken into 2 smaller nuclei Fusion- smaller nuclei join to form a larger, more stable nuclei

Reactants and Products

Nuclear Power- generated by a controlled fission chain reaction Control rods- absorb neutrons to slow the chain reaction Made of cadmium Inserted or withdrawn to keep temp of reaction steady 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

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