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ARCC CHEM 1020 - Chapter 9

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Why is hydrogen reactive and flammable but helium is not? Recall Mendeleev's Periodic Law:When the elements are arranged in order of increasing atomic number, certain sets ofproperties reoccur periodically. WHY???Not only is hydrogen reactive but so are ALL the Group IA elements. Helium and the other Noble Gases are ALL unreactive. To understand the behavior of electrons in atoms, we first have to understand the behavior of light.Elemental properties reoccur periodically because of the behavior of the electrons in those atoms…..Chapter 9: Electrons in Atoms and the Periodic Table Ch 9 Page 1Visible light -the form of electromagnetic radiation that we can "see"Microwaves, radiowaves, x-rays, ultraviolet radiation, infrared radiation - the forms of electromagnetic radiation that we can't see.Electromagnetic radiation travels in waves, like waves move across the surface of a pondLight is one of the forms of energy All electromagnetic radiation travels in waves at 3.00 x 108m/sec(the speed of "light" in a vacuum) and obeys the physics of waves. How fast is 3.00 x 108m/sec?1 sThe Nature of Light -It's A Wave! Ch 9 Page 2All typesof electromagnetic radiation are characterized by their wavelength and frequencyVisible light λλλλ usually measured in nanometers (1 nm = 1 x 10-9m)Wavelength (λλλλ): a measure of the distance covered by a wave (the distance from one crest to the next)The number of waves per second = number of cycles per secondUnits for frequency = hertz (Hz) or cycles/sec (1 Hz = 1 cycle/sec)Frequency (νννν): the number of waves that pass a point in a secondwhen λλλλincreases, ννννdecreases; when λ λ λ λ decreases, ν ν ν ν increasesWavelength and frequency are inversely relatedλ x ν = 3.00 x 108m/secAll electromagnetic radiation obeys the equation:The Electromagnetic Spectrum Ch 9 Page 3Visible light is separated into its constituent colors—red, orange, yellow, green, blue, indigo, and violet—when it is passed through a prism. The color of the light is determined by it's wavelength.V ≅380 nmR ≅750 nmOYGBIThe Visible SpectrumRed light:Blue light:λ= 750 nmλ= 450 nmThe observed color is predominantly the colors reflectedWhen an object absorbs some of the wavelengths of white light while reflecting others, it appears colored. Visible Light and Color Ch 9 Page 4Scientists in the early 20thcentury showed that electromagnetic radiation was composed of particles we call photons, giving rise to the wave-particle duality of electromagnetic radiation. Each wavelength has photons that have a different amount of energy What is the relationship between energy and wavelength?Write the order of the following types of electromagnetic radiation: microwaves, gamma rays, green light, red light, ultraviolet lightBy wavelength (short to long)•By frequency (low to high)•By energy (least to most) •The Nature of Light - It's a Particle! Ch 9 Page 5If white light is passed through a prism (or diffraction grating) it separates into a continuous spectrum - a rainbow of colors.If an element is heated enough so that it gives off light and that light is passed through a diffraction grating, only narrow, colored lines are observed - called aLINE SPECTRUM.The wavelengths of light emitted depends on the element.Hg(g) H2(g)Atomic Line Spectra Ch 9 Page 6Whyare only certain colors of light given off by glowing elements?The Bohr modelcan help explain why:Electrons orbit at specific fixed distances from the nucleus, called shellsThe energy of each shell is quantized, which means that an electron in a particular shell can only have a very specific energy. (The energy of a shell is specified by the quantum number n, where n = 1, 2, 3, etc).The farther the electron is from the nucleus the more energy it has.Quantized energy levels in the Bohr model of the atom are like steps on a ladder: you can stand on one step or another, but you can't stand in between. Bohr Model of the Atom Ch 9 Page 7Electrons jumpto higher energy shells when they absorbenergy (called "excitation")Electrons dropdown to lower energy shells and theygive offadefined amount of energy (called "relaxation")The defined amount of energy given off during an electron's transition to a lower energy shellcorresponds to the emission of a photonof a particular wavelength of light (i.e. n= 2 to n=1, n=4 to n=2, etc).The closer the shells are in energy, the lower the energy of the photon emitted (Lower energy = longer wavelength)The Bohr model helped explain the SHARP LINE SPECTRA of the elements and introduced the idea of quantization of energy levels. Bohr Model of the Atom Ch 9 Page 8Can predict the energies of the shells in a hydrogen atom, but not for any other elementAssumes electrons travel in single circular paths, which doesn't fit with the known laws of physics Does not explain the fact that certain sets of element properties reoccur periodically Shortcomings of the Bohr model:However, the Bohr model of the atom did introducequantization of energy levels, which was correct.Erwin Schrödinger treated electrons as matter wavesand applied the mathematics of probability to the physics equations that describe waves.Solutions to the 'Wave Equation' describe the probability of finding an electron within a certain region of space called an ORBITALQuantization of energy levels came naturallyfrom solutions to the 'Wave Equation' Assumes electrons can act like particles or wavesQuantum mechanical model:Exact path of a single electron is impossible to predict with the quantum mechanical modelErwin SchrödingerThe Quantum-Mechanical Model of the Atom Ch 9 Page 9Orbital: 3-dimensional region around the nucleus where an electron is most likely to be found 90% of the timen= 1, 2, 3, 4, … (principal quantum number)•As nincreases, energy of the electron increases, and average distance from the nucleus increases.•The quantum mechanical model has shellslike the Bohr modelBut shells alone cannot explain electron behavior. Each new row on the periodic table is a new shell, and the major sections (main group, transition, inner transition) each have their own subshell.Each shell has nsubshells, with each subshell within a shell having a slightly different energy (s < p < d < f)1s orbital 1s orbital electron probability mapShell# subshellsSubshell lettersn = 1 11sn = 2 22s, 2pn= 3 33s, 3p, 3dn= 4 44s,


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