Chapter 9 ELECTRONS IN ATOMS AND THE PERIODIC TABLE Problems 1 3 13 15 19 23 25 31 32 43 45 46 49c 50a 50b 57c 58 b c d 61 62 69 71 74 77 88 91 94 9 5 LIGHT Electromagnetic Radiation Light is a form of electromagnetic radiation a type of energy that travels through space at a constant speed known as the speed of light symbol c 2 998 108 m s 186 000 mi hour While light may appear instantaneous to us it s really a wave traveling at this finite speed The term electromagnetic comes from the theory proposed by Scottish scientist James Clerk Maxwell that radiant energy consists of waves with an oscillating electric field and an oscillating magnetic field which are perpendicular to one another 9 3 Electromagnetic Spectrum continuum of radiant energy see Fig 9 4 on p 280 The substances below are about the size of the wavelength indicated in the EM spectrum e g an atom is about 10 10 10 9 m in size while a CD is about 10 3 m or 1 mm thick visible region the portion of the EM spectrum that we can perceive as color For example a red hot or white hot iron bar freshly removed from a high temperature source has forms of energy in different parts of the EM spectrum red or white glow falls within the visible region heat falls within the infrared region CHEM 121 Tro Chapter 9 v1012 page 1 of 13 Thus these electromagnetic waves have both a wavelength and a frequency wavelength Greek lambda distance between successive peaks frequency Greek nu number of waves passing a given point in 1 s How is energy related to wavelength and frequency As the wavelength the frequency and the energy As the wavelength the frequency and the energy Example Which is higher in energy red light at 700 nm or blue light at 400 nm CHEM 121 Tro Chapter 9 v1012 page 2 of 13 Classical Descriptions of Matter John Dalton 1803 Atoms are hard indivisible billiard like particles Atoms have distinct masses what distinguishes on type of atom from another All atoms of an element are the same JJ Thomson 1890s discovered charge to mass ratio of electrons atoms are divisible because the electrons are one part of atom Ernest Rutherford 1910 shot positively charged alpha particles at a thin foil of gold discovery of the atomic nucleus James Maxwell 1873 visible light consists of electromagnetic waves Transition between Classical and Quantum Theory Max Planck 1900 Blackbody Radiation heated solids to red or white heat noted matter did not emit energy in continuous bursts but in whole number multiples of certain well defined quantities matter absorbs emits energy in bundles quanta single bundle of energy quantum Albert Einstein 1905 Photoelectric Effect Photoelectric Effect Light shining on a clean metal emission of electrons only when the light has a minimum threshold frequency 0 For 0 no electrons are emitted For 0 electrons are emitted more e emitted with greater intensity of light Einstein applied Planck s quantum theory to light light exists as a stream of particles called photons CHEM 121 Tro Chapter 9 v1012 page 3 of 13 9 4 The Bohr Model Atoms with Orbits A Danish physicist named Niels Bohr used the results from the hydrogen emission spectrum to develop a quantum model for the hydrogen atom Bohr Postulates Bohr Model of the Atom 1 Energy level Postulate Electrons move in discrete quantized circular orbits around the nucleus tennis ball and stairs analogy for electrons and energy levels a ball can bounce up to or drop from one stair to another but it can never sit halfway between two levels Each orbit has a specific energy associated with it indicated as the principal energy level or quantum number n 1 2 3 ground state or ground level n 1 lowest energy state for atom when the electron is in the lowest energy level in a hydrogen atom excited state when the electron is in a higher energy level n 2 3 4 2 Transitions Between Energy Levels When an atom absorbs energy the electron can jump from a lower energy level to a higher energy level When an electron drops from a higher energy level to a lower energy level the atom releases energy sometimes in the form of visible light CHEM 121 Tro Chapter 9 v1012 page 4 of 13 Emission Spectra continuous or line spectra of radiation emitted by substances a heated solid e g the filament in an incandescent light bulb emits light that spreads out to give a continuous spectrum spectrum of all wavelengths of light like a rainbow Hydrogen Line Spectrum In contrast when a sample of hydrogen is electrified the resulting hydrogen emission spectrum contains only a few discrete lines These discrete lines correspond to specific wavelengths specific energies The hydrogen atoms electrons can only emit certain energies The energy of the electrons in the atom must also be quantized Planck s postulate that energy is quantized also applies to the electrons in an atom Each element has a unique line spectrum Emission spectra can be used to identify unknown elements in chemical analysis The element s line spectrum is often called its atomic fingerprint CHEM 121 Tro Chapter 9 v1012 page 5 of 13 Other examples of emission line spectra for mercury and neon to compare with hydrogen 9 6 THE QUANTUM MECHANICAL ORBITALS Limitations of the Bohr Model Quantum Mechanical Model Unfortunately the Bohr Model failed for all other elements that had more than one proton and more than one electron The multiple electron nuclear attractions electron electron repulsions and nuclear repulsions make other atoms much more complicated than hydrogen Quantum Mechanical Model In 1920s a new discipline quantum mechanics was developed to describe the motion of submicroscopic particles confined to tiny regions of space Quantum mechanics makes no attempt to specify the position of a submicroscopic particle at a given instant or how the particle got there It only gives the probability of finding submicroscopic particles e g food court analogy Instead we take a snapshot of the atom at different times and see where the electrons are likely to be found See Fig 9 16 on p 296 CHEM 121 Tro Chapter 9 v1012 page 6 of 13 ORBITALS AND THEIR SHAPES Erwin Schr dinger 1926 developed a model that predicts the probability of finding the electron near a given point probability density for an electron is called the electron cloud shape of atomic orbitals Energy Levels and Sublevels For all other elements with more than 1 proton and more than 1 electron principal energy levels numbered 1 2 3 are further divided into energy sublevels s p d f Principal Energy Level n 1 2