The Atom. Unit 3 Atomic Structure And Nuclear Chemistry. Ancient Ideas of the Atom. Ancient Ideas of the Atom. Ancient Ideas of the Atom

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1 1 The Atom Unit 3 Atomic Structure And Nuclear Chemistry What are the basic parts of an atom? How is an atom identified? What is nuclear chemistry? How is a nuclear equation written? Atom Smallest particle of an element that retains the properties of that element As small as ~0.5Å (angstroms) 5 x m Microscopes cannot see much inside the atom Ancient Ideas of the Atom Democritus Ancient Greece 400 B.C. First person credited with idea of atom Ancient Ideas of the Atom Democritus ideas: All matter made of Atoms: Move through empty space Solid, homogeneous, indestructible, and indivisible Different atoms have different shapes and sizes Size, shape, and movement determine properties Ancient Ideas of the Atom Aristotle Greek philosopher Everything made of 4 elements: Fire Hot Air light Earth cool, heavy Water wet Blend those 4 in different proportions Ancient Ideas of the Atom No scientific evidence to back up either Democritus or Aristotle People believed Aristotle based on reputation Aristotle s theory persisted for 2,000 years

2 2 John Dalton 1803 Summarized results of his experiments and those of others 1. All matter made of tiny indivisible atoms 2. Atoms of same element are identical, atoms of different elements are different 3. Atoms of different elements combine in whole number ratios to form compounds 4. Chemical reactions involve rearrangement of atoms No atoms created or destroyed Billiard Ball Model 1 st model of the atom John Dalton Atoms are small, solid spheres Law of Definite Proportions Joseph Proust All compounds contain the same elements in the same ratio Law of Multiple Proportions John Dalton Same atoms can make different compounds Ex: CO 2 vs. CO Law of Conservation of Mass Antoine Lavoisier Matter is neither created nor destroyed in chemical reactions

3 3 Parts of Atoms Parts of Atoms Electron Cathode Ray Tube Experiment J. J. Thomson 1897 Passed current through vacuum tube All air has been pumped out Discovered Electrons Negatively-charged particles Plum Pudding Model JJ Thomson 1904 Negatively charged electrons reside in uniform positive charge Like chocolate chips in a cookie Uniform Positive Charge Proton Ernest Rutherford 1919 Positively charged pieces 1840 times heavier than the electron Parts of Atoms Neutron James Chadwick 1932 No charge Same mass as a proton Parts of Atoms Rutherford s Experiment Gold Foil Experiment Ernest Rutherford 1910 Tested the plum pudding model Rutherford s Experiment Gold Foil Experiment Shot Alpha Particles at gold foil Positively charged pieces Helium atoms minus electrons Screen glows when particles hit

4 4 Rutherford s Experiment Gold Foil Experiment Hypothesis: Particles will pass through without much direction change Atom s positive charge is spread out evenly and has little effect on particles Rutherford s Experiment Gold Foil Experiment Results: Most particles passed through Some particles moved a lot Positive pieces were heavy Nucleus Dense, positively charged center of atom Later found to have protons and neutrons Nuclear Atomic Model Nuclear Atomic Model Rutherford 1911 Mostly empty space Dense, positive nucleus at center Surrounded by electrons Bohr Model Bohr s Ring Model AKA: Planetary Model Niels Bohr 1913 Nucleus at center Electrons are in circular energy levels in the atom Calculated energy released when electrons drop energy levels Only works for hydrogen The Quantum Mechanical Model Quantum Mechanical Model Erwin Schrödinger Electron location is described as probability Current model More in Unit #4 Proton Neutron Electron Atoms Relative Name Symbol Charge mass p + n 0 e Location in Atom Nucleus Nucleus Electron Cloud

5 5 Atoms Atomic Number Number of protons in nucleus All protons are alike Same as number of electrons in neutral atom Determines atom s identity Atoms Mass Number Protons + Neutrons Essentially all mass of atom Note: DO NOT ROUND NUMBER ON PERIODIC TABLE! Will be given to you or determined from problem Atoms Protons Equal to Atomic Number Neutrons Mass Number Atomic Number Electrons Equal to atomic number in neutral atom Isotopes Isotopes Atoms of the same element with: Different numbers of neutrons Different mass numbers Describe one specific atom with a specific mass Isotopes Isotopes Isotopes To name: Always include mass number Put mass number after name of element Carbon 12, Carbon 14, etc. Element Symbols Symbol of the element from table Note: second letter is ALWAYS lowercase Mass number on top Atomic number on bottom Optional unless complete symbol Mass Number Atomic Number X

6 6 Ions What if atoms aren t neutral? Ions Charged atoms Result from loss or gain of electrons Charge = p + e - Ions Anion Negatively charged ion Result from gaining electrons Add charge to electron total (ignore negative sign) Ions Cation Positively charged ion Result from loss of electrons Subtract charge from electron total Atomic Mass Atomic Mass Decimal numbers on the periodic table Weighted average of all isotopes of an element Based on abundance of each isotope in nature Atomic Mass Atomic Mass Too small to be measured in grams Atomic Mass Unit (amu) Mass of 1 proton or 1 neutron 1/12 th the mass of a 12 C atom Atomic Mass Atomic Mass - Calculating Unless told otherwise, mass of isotope is mass number in amu Convert percent abundance to decimal Divide by 100 Multiply isotope mass by decimal for each isotope, then add the results

7 7 Isotopes Different numbers of NEUTRONS Some isotopes more stable than others Band of Stability Shows ratios of protons to neutrons in a stable nucleus Radioisotope Isotope with unstable nucleus /Radioactive Decay Spontaneous change radioisotopes undergo to become stable Shown in nuclear equation Mass and atomic numbers MUST balance on both sides Parent is original atom, daughter atom is result /Radioactive Decay Atoms release particle and/or electromagnetic radiation (EMR) ALWAYS results in a more stable nucleus ALWAYS results in a new element Transmutation The change in identity of the element after it undergoes radioactive decay Alpha particle (α) Helium nucleus 2 protons, 2 neutrons Release makes nucleus smaller Relatively low energy, heavy particles Easily shielded by paper or clothing Beta Particle (β) Result of a neutron breaking down 1 neutron is converted to 1 proton and 1 electron Proton stays in nucleus Beta particle is the electron More energy than alpha, but still easily shielded by Al foil or wood Gamma Radiation (γ) High energy radiation No mass or charge Released along with alpha or beta particles Must be shielded using lead or concrete

8 8 Half Life All radioisotopes decay at a constant rate No two radioisotopes decay at the same rate Half Life (t 1/2 ) Time required for half the atoms of radioisotope to decay Can be a few seconds to billions of years Radiation Detection Film Badges Exposure of film measures radiation exposure Geiger Counters Detect radiation through electric pulses in ionized gas Scintillation Counters Measure radiation from substances that emit visible light when energy is absorbed Uses of Radioactive Dating Determine approximate age of a fossil Medical Field Detect and kill cancerous cells X-Rays Disinfect Foods (Food Irradiation) Smoke Detectors Nuclear Fusion Fusion Nuclei combine to make nucleus with greater mass Releases a LOT of energy Energy produced by the sun Nuclear Fission Fission Radioisotope bombarded by neutrons Splits into smaller pieces Releases large amount of energy Less than fusion Can result in a chain reaction Criticality Nuclear Energy Nuclear Power Plant Fission creates heat, which boils water Steam spins turbines, creating electricity Water is then cooled off Same process in nuclear submarines Nuclear Fission produces about 20% of energy in US