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Chapter 7: Quantum theory of the atom

Chapter 7: Quantum theory of the atom. Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor. Atomic emission and line spectra. When different compounds are burned, they give off surprisingly different colors of light It can be used to identify certain compounds

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Chapter 7: Quantum theory of the atom

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  1. Chapter 7: Quantum theory of the atom Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor

  2. Atomic emission and line spectra • When different compounds are burned, they give off surprisingly different colors of light • It can be used to identify certain compounds • If the emitted light is sent through a prism so the colors are separated, only certain discrete colors of light are given off (atomic line spectrum) • The color of light can be related to the amount of energy that light contains

  3. The wave nature of light • Electromagnetic radiation: energy that is in the form of a wave, (visible light, x-rays, radio waves, etc) • Wavelength, : distance between any two adjacent identical points of a wave • Visible light, wavelength measured in nm • Radio waves can be measured in m • Frequency,  (nu): number of wavelengths that pass a fixed point in one unit of time (usu. 1 second)

  4. Electromagnetic spectrum

  5. Frequency and wavelength • All electromagnetic waves travel at the speed of light, c = 3.00 x 108 m/s • c = , if  is in m, and  is in sec-1 • Visible light wavelengths are always given in nm, between 400 and 800 nm • Frequency is usually given in sec-1, or Hz

  6. The particle nature of light • While light has wave-like properties, it also has particle-like properties • Photon: discreet particles of energy which make up light (or any electromagnetic radiation) • The energy of one photon of light is related to the frequency of that light E = h • (where h is Planck’s constant, 6.63x10-34 J·s) • This relates the wave-like and particle-like properties of light

  7. More about atomic line spectra • Heated solid metals emit light of all wavelengths, or a continuous spectrum • Would form a rainbow if sent through a prism • Heated gases emit light of only particular wavelengths, or a line spectrum • Would form only lines of particular colors if sent through a prism • These lines are associated with energy level transitions

  8. Energy levels • Electrons can have only specific energy values in an atom (energy levels) • Energy levels are quantized (only specific allowed values) • When an electron absorbs energy from the environment, it can be promoted to a higher energy level • In order for it to return to a lower level, energy must be released in the form of a single photon • Depending on which levels this transition involves, the photon will have a different amount of energy

  9. H atom energy level calculations • Energy levels are numbered with integers starting with 1, symbol is n n = 1, 2, 3, … • The energy of a particular level is given by E = -(RH) / (n2) where RH = 2.179 x 10-18 J • The energy of a photon given off can be calculated by subtracting the lower energy level from the higher energy level (energy of a photon is positive)

  10. Quantum mechanics • Just like light can be wave-like and particle-like, so can electrons • The most accurate description of an electron’s behavior is using a wave-like interpretation, this is known as quantum mechanics • An electron can be described by a wavefunction – an equation for the wave that represents an electron • Only the probability of an electron appearing in a certain place can be calculated • Heisenberg uncertainty principle says the more precisely you know the position of a small particle, the less precisely you know its momentum

  11. Atomic orbitals • The 3-dimensional space in which there is a high probability of finding an electron in an atom is referred to as an atomic orbital • Can be described by three quantum numbers • Principal quantum number, n: refers to the energy of an electron, it also associates with the size of an orbital (n = 1, 2, 3, 4,…)

  12. Atomic orbitals • Angular momentum quantum number, l: indicates shape of orbital (l= 0, 1, 2, 3, …. n-1) • Usually shown by letters: s, p, d, f, and g • Magnetic quantum number, ml: Distinguishes orbitals of same shape but different position (ml = integers from –l to +l) • Spin quantum number, ms: indicates which of 2 possible spin states an electron is in, equal to either -1/2 or +1/2

  13. Permissible atomic orbitals for n = 1, 2, 3

  14. Atomic orbital shapes

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