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Development of Atomic Models
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Democritus Greek philosopher 400 BC “Atomos” concept
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Can matter can be divided forever?
Eventually, a piece would be “indivisible” “Atomos,” meaning “not to be cut,” is smallest piece of matter
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John Dalton (early 1800’s) Coined the term “atom”.
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Dalton’s Atomic Theory
Matter made of tiny indivisible particles called “atoms”. Atoms of one element are alike, and different from atoms of other elements.
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Page from Dalton’s Journal
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Compounds form when different atoms combine in fixed proportions.
Chemical reactions involve rearrangement of atoms. Atoms can’t be created or destroyed, but are conserved in chemical reactions.
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Dalton’s Atomic Theory called
“Hard Spheres Model”
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JJ Thomson (1897)
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Thomson’ Experiments Studied “cathode rays” (electric current) in a “Crooke’s Tube”. Fluorescent screen, shows how cathode ray behaved in a magnetic field. Lets draw a typical Crooke’s Tube in our notes.
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Cathode Rays were negatively charged
Cathode Ray Tube and Magnet Cathode Rays were negatively charged They bent toward (+) plate
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Cathode Rays were particles
They couldn’t pass through matter.
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JJ is Awesome Concluded the negative “cathode ray” particles came from within atoms. Discovered first subatomic particle (electron).
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What about the Positive?
But…matter is neutral. Therefore: A positive charge must exist to balance the negative.
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Plum Pudding Model Atoms are positively charged spheres with negatively charged particles scattered throughout.
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Yummy… Brian Cox: Thompson and Discovery of Electron
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Ernest Rutherford (1908) Physicist who worked in new field of radioactivity.
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Found 3 Different Types of Radiation
Used magnetic field to isolate three types of radiation. Alpha (α) Beta (β) Gamma (γ)
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Identify the charge each type of radiation has.
Charges of Radiation The radiation had different charges. Identify the charge each type of radiation has.
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Gold Foil Experiment Shot alpha particles, at very thin piece of gold foil. Alpha particles have a positive charge, and a mass of 4 amu Fluorescent screen shows where the particles went. Rutherford Gold Foil
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Observation: Most alpha particles passed straight through gold foil. Conclusion: Atom’s volume is mostly empty space.
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small, dense positively charged nucleus.
Observation: A few alpha particles deflected at an angle or bounced back. Conclusion: Atoms have a very small, dense positively charged nucleus.
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Deflections happened rarely (1/8000).
Nucleus is extremely small compared to the size of the atom as a whole. Deflections happened rarely (1/8000). Modern Example of Gold Foil Experiment in Action
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The Nuclear Model Rutherford’s Model is called the “Nuclear Model”
Brian Cox: Rutherford and the Nucleus
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Comparison to Thomson Positively charge only contained in nucleus.
Negatively particles scattered outside nucleus. Charge is not disbursed evenly.
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Niels Bohr (1913) Came up with the “Planetary Model”
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Bohr’s Theory Electrons circle nucleus in specific energy levels or “shells”. The higher the “energy level,” the higher the electron’s energy.
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Energy Levels Different energy levels can contain different numbers of electrons.
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2n2 = maximum number of electrons an energy level can hold.
How many per level? n = the number of the energy level 2n2 = maximum number of electrons an energy level can hold. Ex: Level 3 can hold 2(3)2 = 18 electrons
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Draw a Bohr Atom Ex: The Fluorine Atom (F) Protons = 9 Neutrons = 10
Electrons = 9 How many energy levels do you draw? How many electrons in each level? Human Bohr Model
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Draw a Bohr Ion They only difference is that one or more electrons gets added or taken out of the outer energy level. Ex: The Magnesium Ion (Mg+2) Protons = 12 Neutrons = 12 Electrons = 10
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(+) Ions (cations) (+) ions are smaller Lost electron(s)
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(-) Ions (anions) (-) ions are larger Gained electron(s)
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How Did Bohr Come Up With His Model?
Studied the spectral lines emitted by various elements (especially Hydrogen)
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What are Spectral Lines?
Energy gets absorbed by an atom causing it to emit a unique set of colored lines. Used to identify what elements are present in a sample. (elemental “Fingerprint”)
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Spectral Lines are Different for Each Element
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Answer: 1
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What Causes Spectral Lines?
Jumping Electrons!! Video of Line Spectra of Hydrogen
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Jumping Electrons Electrons normally exist in the lowest energy level possible called the “ground state”. (stable) “Ground state” e- configurations are written on the periodic table for each element. Ex: Aluminum is 2-8-3 Calcium is
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An Electron Gets “Excited”
Electrons can absorb a photon (or “quanta”) of energy and “jump up” to a higher energy level farther from the nucleus. This is called the “excited state”. (unstable)
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Jumping Electrons They quickly “fall back down” to the ground state. (stable) They emit a photon (or “quanta”) of energy that corresponds to how far they jumped. Spectral Lines
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This photon of energy is seen as a spectral line!
Each spectral line corresponds to a specific photon of energy that is released. Model Of Hydrogen Atom and Electrons Jumping
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REMEMBER Absorb Energy Jump Up Emit Energy Fall Down
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Excited vs Ground State
Periodic table lists ground state electron configurations for neutral atoms. To recognize an “excited state” configuration, count the electrons and see if the configuration matches the one on the table. Ex: = 20 electrons Calcium (atomic # 20) is So this must be showing one of the ways calcium could be in the excited state.
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Valence Electrons Electrons in highest occupied energy level.
Involved in forming bonds with other atoms. Atoms are most stable when they obtain a “stable octet” of 8 valence electrons Noble Gases: (Group 18) Have stable octet already and are “inert” and unreactive Ex: Argon 2-8-8, Neon 2-8
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Valence Electrons Look at the last number in the atom’s electron configuration to determine the number of valence electrons. Ex: Al valence Ca valence F valence
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Lewis Dot Diagrams Shows the number of valence electrons an atom has as “dots” around the atom’s symbol. Phosphorus is 2-8-5
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Kernel Nucleus and non-valence electrons
Inner part of atom not involved directly in reactions Ex: Al has 10 kernel electrons and 3 valence electrons
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The Nature of Light Study of light has provided important information about the structure of atoms. Dual Nature of Light: behaves as both waves and as particles (depending on what type of experiment is being performed.) Speed of Light: all light waves travel at the same velocity C = 3.0 x 108 meters/sec
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Electromagnetic Spectrum
Spectral lines can come from all areas of the EM Spectrum. Lines of visible colors make up only a small part of the spectrum.
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Which wave has higher energy?
EM waves carry different amounts of energy based upon their wavelength and frequency. Wavelength (λ): distance between two peaks of a wave Frequency (γ): number of peaks that pass per second. (Hertz (Hz) or cycles/sec) Which wave has higher energy?
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Relationship of Frequency, Wavelength and Energy of colored line
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Good Overview Videos Crash Course: History of Atomic Theory
Quantum Mechanics and the Bohr Model
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Calculating the Energy of a Spectral Line (HONORS)
STEP 1: If you know the wavelength of the spectral line you can find it’s frequency. c = λ x ү c = the speed of light = 3 x 108 meters/sec λ = wavelength (in meters) ү = frequency of the wave
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Calculating the Energy of a Spectral Line (HONORS)
STEP 2: Using the frequency find the energy of the line (in Joules) E = h x ү E = energy in Joules h = Planck's constant = 6.63 × kg x m2 / sec ү = frequency of the wave
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