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Chapter 4 Arrangement of Electrons In Atoms Properties of Light You can treat it three ways • Light as a wave – Diffraction – Interference • Light as a particle – Photoelectric effect • Dual Nature of Light – light can behave as both a wave and a particle. – Electromagnetic Radiation – energy that travels through space as a wave Electromagnetic Spectrum Wave Diagram Wave Mechanics • Wavelength – l – distance between corresponding point on adjacent waves (m) • Frequency – n – (f) – number of waves that pass a certain point in a given time (waves/s) or (/s, s-1) or (Hz) • Speed of a wave = v = ln – For light c=ln (c=3x108 m/s) Proof Light is a Wave • Diffraction – bending of a wave around a barrier. • Interference – combining of waves that cross paths (superposition). Proof Light is a Particle • Photoelectric effect – emission of electrons from a metal when the metal is struck by certain frequencies of light. • Ean • E = hn – h = 6.626x10-34 Js – Plank’s Constant Hydrogen Emission Spectrum E=hn Niels Bohr – explained the spectral lines observed in excited gases c=ln n=c/l E=hc/l Balmer, Paschen, and Lyman Series The DeBroglie Hypothesis E = hn and E = mc 2 If light can behave as both a wave and a particle, can electrons also have this dual nature? hn = mc 2 for slower vel ocities v = c hn = mv 2 hc = mv 2 l hv l = mv 2 hv h = mv 2 mv mv = momentum = p h l= p l= The Quantum Model Heisenberg’s Uncertainty Principle – it is impossible to know both the exact position and the momentum (velocity) of a small particle at the same time. Schrodinger’s Wave Equation – describes the probability of finding an electron at some distance from the nucleus in terms of the wave function Y Implications of Heisenberg and Schrodinger • These ideas say it is impossible to know where an electron is at any point in time. Therefore we can only say where an electron is most probably located at any time. We call that region an orbital. Orbital – 3d region around a nucleus where an electron is likely to exist Quantum Numbers – 4 numbers used to describe the location of an electron • Principle Quantum Number – (n) – tells the main energy level of the electron. • Angular Momentum Quantum Number – (l) – describes the shape of the orbital. • Magnetic Quantum Number – (m) – tells the orientation of the orbital around the nucleus. • Spin Quantum Number – (s) – indicates the direction of the spin of the electron on its own axis. Pauli’s Exclusion Principle – No two electrons have the same set of 4 quantum numbers • Possible values for the quantum numbers – n = 1,2,3,…7 max # of e- in energy level =2n2 – l = n-1 l = 0,1,2,…6 or s,p,d,f,g… – m = (-l,…0…+l) – s = +/- 1/2 Principle Quantum Number Tells the main energy level (how far from the nucleus) of an electron #e-/energy level = 2n2 Angular Momentum Quantum Number – Azimuthal Quantum Number • Tells the type (shape) of the orbital Magnetic Quantum Number – tells orientation around the nucleus Spin Quantum Number s = -1/2 s = +1/2 Electron Configurations – shorthand way of representing the arrangement of electrons in an atom • Pauli’s Exclusion Principle – no two electrons have the same set of four quantum numbers (everybody’s different) • Aufbau Principle – electrons occupy the lowest possible energy level (electrons are lazy) • Hund’s Rule – orbitals of equal energy are occupied by one electron before any one orbital is occupied by two electrons, and all electrons in singly occupied orbitals have the same spin (everybody gets one before anybody gets two) Order of Orbital Filling Order of Orbital Filling Electron Configurations for 1st Period Helium ?? 2+ Notations for 2nd and 3rd Periods Orbital Notation Orbitals Notations for 3p’s Periodic Table with Electron Configurations Noble Gas Notations Here are some examples: O 1s22s22p4 Si 1s22s22p63s23p2 Ca 1s22s22p63s23p64s2 Cr 1s22s22p63s23p63d54s1 Br 1s22s22p63s23p63d104s24p5 La 1s22s22p63s23p63d104s24p64d104f1 5s25p66s2 O [He]2s22p4 Si [Ne]3s23p2 Ca [Ar]4s2 Cr [Ar]3d54s1 Br [Ar]3d104s24p5 La [Xe]4f 16s2. Homework • Pages 124-126 • Numbers 6,10,11,14,18,19,22,30,31,32,33,35,37,38, 46,48,50