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Transcript

Chapter 4 Arrangements of Electrons Properties of Light as a Wave Electromagnetic radiation – form of energy that exhibits wave-like behavior as it travels through space Measurements Involving Waves Wavelength (m, cm, nm) – the distance between two consecutive peaks in a wave. Symbol = l (lambda) Frequency (Hz, 1/s) – the number of waves that pass a given point in a specific time, usually one second. Symbol = n (nu) Wavelength Electromagnetic Spectrum Visible Spectrum R O Y G B I V Indigo has been dropped so now it’s: ROY G. BV Relationships All electromagnetic radiation travels at the speed of light: 3x108m/s Or 186,000 mps Mathematical relationship between frequency and wavelength: c = ln (c=speed of light) Electromagnetic Waves http://www.colorado.edu/physics/2000/waves_particles/index.html Relationships Frequency, Wavelength, and Energy Frequency and Wavelength increases, As the frequency of a wave the wavelength decreases. Therefore these properties are inversely related. Frequency and Energy As the energy of a wave the frequency increases, increases. Therefore these properties are directly related. Wavelength and Energy These properties must be inversely related. http://www.ifae.es/xec/phot2.html Photoelectric Effect Photoelectric effect Applet http://zebu.uoregon.edu/2000/ph101/lec06.html Refers to the emission of electrons from a metal when light shines on the metal. Light had to be a certain wavelength to cause electrons to be emitted. This caused scientists to question the wave nature of light. Properties of Light as a Particle Max Planck – Energy is emitted in small, specific amounts called quanta. Quantum - minimum quantity of energy that can be lost or gained by an atom. Photon – particle of electromagnetic radiation having zero mass and carrying a quantum of energy. E = hn Energy = Planck’s constant (6.626x10-34Js) times frequency Wave-Particle Duality Einstein – electromagnetic radiation (light) exhibits both wave and particle behavior. While light exhibits many wavelike properties, it can also be thought of as a stream of particles. Energy and Electrons Bohr model of the atom the planetary model where the nucleus is orbited by electrons at different energy levels like planets around the sun Ground state lowest energy state of an atom (electrons occupy lowest energy level available) Excited state state in which an atom has a higher potential energy than it has in its ground state (electrons are in higher levels when lower levels are available) Energy Transitions When a quantum of energy hits the atom an electron can jump energy levels to an excited state. When an excited electron falls back down from its excited state to the ground state, it emits a quantum of electromagnetic energy equal to the amount that it took to get it there. Energy absorbed Energy emitted Excited Ground State Emission Continuous spectrum – the complete electromagnetic spectrum including all forms of energy. (Ex: rainbow) Line-emission spectrum – when an atom is energized and the light given off is passed through a prism, a series of light with specific energy is emitted. Elemental Line Emission Spectrums Hydrogen Line emissions Location of Electrons Orbital Electrons, like light, exhibit both particle and wave-like behavior!!! Heisenberg Uncertainty Principle – it is impossible to simultaneously measure both the location and velocity of an electron. Since we can’t know exactly where an electron is at any specific time or how it travels, then we can only say where it probably is. This 3-d region is known as an orbital. Orbital – three-dimensional region around the nucleus that indicates the probable location of an electron. Can hold a maximum of two electrons. Quantum Numbers Address of an Electron 1. Principle Quantum Number (n) Corresponds to the energy level (shell) Relative cloud size Is there a pattern? 8e2e- 18e- The maximum number of electrons possible in an energy level = 2n2 Practice Problems: Calculate the maximum number of electrons possible in the following: n=2 #e- = 2(22) = 8 n=3 #e- = 2(32) = 18 n=7 #e- = 2(72) = 98 n=9 #e- = 2(92) = 162 2. Angular Momentum Quantum Number (l) Corresponds to the sublevel. Cloud shape The maximum number of sublevels possible in an energy level = n For instance: The first level (n=1) can have 1 sublevel. The second level (n=2) can have 2 sublevels. Etc… **Only four sublevels have been identified.** Symbols and Values for (l) Values: symbol s l = 0 to n-1 l value 0 Remember an orbital is the space that can be occupied by a pair of electrons!! # of orbitals 1 # of e- 2 p 1 3 6 d 2 5 10 f 3 7 14 Shapes Practice Problem How many sublevels can the fourth level (n=4) have? 4 Write the symbols and l values for the four sublevels. s l=0 p l=1 d l=2 f l=3 Review Energy Level (n) Sublevels # of Electrons 1 s 2 2 s, p 2+6 = 8 3 s, p, d 2+6+10 = 18 4 s, p, d, f 2+6+10+14 = 32 Magnetic Quantum Number (m) Refers to the orbital Orientation of the orbital in space (direction) Orbital – the space that can be occupied by a pair of electrons. The values of m = -l to +l Assigning m values The values of m = -l to +l symbol l value # of orbitals m values s 0 1 0 p 1 3 -1, 0,+1 d 2 5 f 3 7 -2,-1, 0,+1, +2 -3,-2,-1, 0,+1, +2, +3 Spin Quantum Number (s) Refers to the spin of the electron CW or CCW Values of s may equal +1/2 or -1/2 Designated by arrows ( or ) Pauli Exclusion Principle – no two electrons in an atom can have the same set of four quantum numbers. Degenerate orbitals – orbitals of the same energy (level & sublevel) Hund’s Rule – shows a maximum number of two electrons in a sublevel. (Electrons do not double up in an orbital unless necessary!) Summary of Quantum Numbers Quantum Number Symbol Represents Information Values 1. Principal n Level Size of cloud n = 1 to … 2. Angular l Sublevel Shape of cloud l = 0 to n-1 3. Magnetic m Orbital Orientation m = -l to +l 4. Spin s Spin CW or CCW s = +½ or -½