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UNIT III - ATOMIC THEORY 1 I. Atomic Basics * atom - from Greek atomos, meaning “indivisible”; first coined by Democritus (300-200 BC) A. Parts of the atom - subatomic particles Particle proton (p+) neutron (no) electron (e-) Approximate Mass (amu) 1 1 1/1837 Charge +1 0 -1 Location nucleus nucleus outside nucleus * 1 atomic mass unit (u) = 1.66 x 10-27 kg B. Relationship to charge - found by balance of charged particles (protons and electrons) - 1 proton + 1 electron = neutral charge (0) * ion - an atom with an unbalanced number of electrons and protons; a charged particle S. What is the charge on an atom containing: ____1) 11p+, 12no, 10e- ____2) 16p+, 16no, 18e- ____3) 25p+, 30no, 23e- C. Atomic Number - defined as the number of protons in an atom of an element - used to identify elements S. Using the three examples above, identify what elements they are ____1) ____2) ____3) D. Mass Number - defined as the total number of protons and neutrons - Warning!!! : do not confuse this with atomic mass ____1) S. Using the three examples above, give the mass number ____2) ____3) UNIT III - ATOMIC THEORY 2 E. Isotope - most elements have two or more different forms with different mass numbers (therefore, different amounts of neutrons), these different forms are called isotopes of that element - when dealing with samples of an element containing two or more different isotopes, the mass number must be indicated with the element’s name: - another way to represent different isotopes is to put the mass number on the top-left of the symbol for that element: magnesium-24 : mass number = 24, protons = , neutrons = Symbol: magnesium-25: , neutrons = Symbol: mass number = 25, protons = * Electrons occur in Energy Levels (areas which can hold up to a certain number of electrons) S. Write the correct symbol and draw a picture of the following atoms: * Also label each as an atom or ion 1) hydrogen-1 2) carbon-14 3) 4) S. Give the full symbol for each of the following elements: 1) 2) 3) 4) UNIT III - ATOMIC THEORY 3 II. History of Atomic Theory - Evolution of the Atomic Model A. John Dalton (1803) - first resurrected term “atom” * What was known at this time? 1) Law of Constant Composition – a given compound always contains the same proportion of elements by mass * example: water is always composed of 88.9% O & 11.1% H 2) Law of Conservation of Mass * 4 parts to his theory: 1) All matter is composed of indivisible, indestructible atoms. 2) All atoms of the same element are similar. 3) All atoms of different elements are different. 4) A compound is a chemical combination of two or more atoms. * Drawing: How did Dalton “see” atoms? B. J.J. Thomson (1897) * Crookes (1870’s) – developed the Cathode Ray Tube * Thomson found cathode rays were bent by electrical and magnetic fields - called them electrons * Drawing: Show Thomson’s “plum pudding” model C. Ernst Rutherford (1909) * performed experiment with alpha particles - apparatus UNIT III - ATOMIC THEORY 4 * What were the results of his experiment? * Why did these results contradict Thomson’s model? * 2 conclusions: 1) Atoms are mostly empty space. 2) In the center of the atom is a densely-packed, positively-charged nucleus. *NOTE: no protons or neutrons yet * Problem with Rutherford’s Model – Where are the electrons? D. Niels Bohr (1913) - electrons are in energy levels; based his concept on Quantam Theory and bright-line spectra for hydrogen 1) Quantam Theory – Max Planck (1900) – an object emits energy in specific quanta (packets) which correspond to their energy 2) Photoelectric Effect – Einstein (1905); purple light caused release of electrons from sodium metal, but red did not light is emitted in quanta – photons * purple photons have greater energy than red ones, therefore electrons escape 3) Bohr’s Model: * ground state – when all electrons are in their lowest possible energy state * excited state – when one or more electrons absorb a quantum of energy, it “jumps” to a higher energy state; in order to return to the ground state, it emits a specific amount of energy (therefore a specific wavelength) * problem with Bohr model only works for hydrogen (one electron) E. Charge-Cloud Model - also called orbital model and quantam-mechanical model 1) Matter as Waves – DeBroglie (1924) * electrons have wavelengths and frequencies like light * so energy has particle property (quanta), and matter has wave property 2) Heisenberg’s Uncertainty Principle – (1927) * the exact position and velocity of an object can not be known simultaneously 3) Quantam-Mechanical Model * basically says electrons are somewhere outside nucleus, not in neat orbits - this “area” is known as an electron cloud * orbital - area around nucleus which contains 2 electrons of opposite spin * Drawing: UNIT III - ATOMIC THEORY 5 III. Atomic Mass - weighted average of all isotopes of a certain element T. Chlorine comes in two isotopes: Chlorine-35 and Chlorine-37. You take a sample of chlorine in nature and find that it contains 75.53% chlorine? and 24.47% S. 1) Bromine occurs in the following proportions: Bromine-79 Bromine-80 Bromine-81 25.34% 50.00% 24.66% What is the atomic mass of bromine? 2) Oxygen occurs in the following proportions: oxygen-16 oxygen-17 oxygen-18 99.76% 0.04% 0.20% What is the atomic mass of oxygen? . What is the atomic mass of UNIT III - ATOMIC THEORY 6 IV. The Nature of Light - behaves as both a wave and particle - we’ll deal with it as a wave: wavelength amplitude peak * frequency – number of peaks that pass a fixed point per second; Energy is directly proportional to frequency (i.e. the higher the frequency of the radiation, the more energy it has) * electromagnetic radiation - the energy of a light beam is directly proportional to the frequency - color of light depends on frequency - visible light - What color light has the highest energy? lowest? * each element releases a bright-line spectra when its atoms are excited UNIT III - ATOMIC THEORY 7 V. Orbital Diagrams - represent where electrons reside * Each energy level is assigned a principal quantam number (n) * Each energy level subdivides into a number of sublevels equivalent to “n”: Principal Energy Level (n) Sublevels Available 1 2 3 4 5 6 1s 2s, 2p 3s, 3p, 3d 4s, 4p, 4d, 4f 5s, 5p, 5d, 5f, 5g 6s, 6p, 6d, 6f, 6g, 6h * Each sublevel has a corresponding energy value: 4f 4d n=4 4p 3d 4s n=3 3p 3s 2p n=2 2s n=1 1s * Each sublevel contains a certain number of orbitals: Principal Energy Level (n) s Number of sublevels available p d f Total Number of Orbitals Total Number of Electrons 1 2 3 4 1 1 1 1 -3 3 3 --5 5 ---7 1 4 9 16 2 8 18 32 UNIT III - ATOMIC THEORY 8 * OK, let’s build some atoms * Rules: 1) 2) 3) 4) 5) T/S. Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Electrons want to be in the lowest possible E level available. Orbitals hold 2 electrons. We are only dealing with neutral, ground state atoms. Pauli Exclusion Principle - 2 electrons in the same orbital must have opposite spin Hund’s Rule - electrons want to spread out as much as possible within a sublevel. 1s 2s 2p 3s UNIT III - ATOMIC THEORY 9 VI. Electron Configurations - a shorter way of representing electron locations 1s2 * pronounced: “one - s - two” * label what each part represents * easily done directly from the periodic table: T. Give the electron configuration for: 1) Hydrogen 2) Helium 3) Lithium 4) Carbon 5) Fluorine UNIT III - ATOMIC THEORY 10 S. Give the electron configuration for: 1) Boron 2) Cobalt 3) Calcium 4) Aluminum 5) Chlorine VII. Advanced Electron Configurations - silly to show lower patterns if they are always the same - example: 1s22s22p6 is Neon. This never changes for any other element of higher number - use [Ne] instead of 1s22s22p6 T. 1) Aluminum 2) Bromine 3) Potassium 4) Promethium 5) Osmium UNIT III - ATOMIC THEORY 11 S. 1) Silicon 2) Krypton 3) Vanadium 4) Holmium 5) Mercury * There is one set of strange ones: columns 6 and 11 - half-filled orbitals T. 1) Chromium 2) Copper S. 1) Molybdenum 2) Silver UNIT III - ATOMIC THEORY 12 VIII. Orbital Shapes A. s-orbitals B. p-orbitals C. d-orbitals D. f-orbitals How many lobes would you guess they have? _______