Chapter 6 Characteristics of Atoms

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Chapter 6Characteristics of Atoms

• Department of Chemistry and Biochemistry
• Seton Hall University

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Characteristics of Atoms

• Atoms posses mass
• most of this mass is in the nucleus
• Atoms contain positive nuclei
• Atoms contain electrons
• Atoms occupy volume
• electrons repel each other, so no other atom can
  penetrate the volume occupied by an atom
• Atoms have various properties
• arises from differing numbers of protons and
  electrons
• Atoms attract one another
• they condense into liquids and solids
• Atoms can combine with one another

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Wave aspects of Light

• Most useful tool for studying the structure of
  atoms is electromagnetic radiation
• Light is one form of that radiation
• Light is characterized by the following
  properties
• frequency, ?, nu
• wavelength, ?, lambda
• amplitude

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Electric and magnetic field components of plane
polarized light

• Light travels in z-direction
• Electric and magnetic fields travel at 90 to
  each other at speed of light in particular medium
• c ( 3 1010 cm s-1) in a vacuum

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Connections between wavelength and frequency

• c 3?108 m/s in a vacuum
• make sure the units all agree!

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Characterization of Radiation
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Wavelength and Energy Units

• Wavelength
• 1 cm 108 Å 107 nm 104 ? 107 m?
  (millimicrons)
• N.B. 1 nm 1 m? (old unit)
• Energy
• 1 cm-1 2.858 cal mol-1 of particles
• 1.986 ? 1016 erg molecule-1 1.24 ? 10-4 eV
  molecule-1
• ?E (kcal mol-1) ? ?(Å) 2.858 ? 105
• E(kJ mol-1) 1.19 ? 105/?(nm)297 nm 400 kJ

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The photoelectric effect

• A beam of light impacts on a metal surface and
  causes the release of electrons (the
  photoelectron) if certain conditions are
  satisfied
• Conditions
• light must have a frequency above the threshold,
  ?o
• number of photoelectrons increases with light
  intensity, but not the kinetic energy

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Explanation of the photoelectric effect

• Ephoton h?photon
• h Plancks constant 6.626 ? 10-34 J s
• Applying the Law of the Conservation of Energy
• energy of the photon is absorbed by the metal
  surface and is transferred to the photoelectron
• the minimum frequency is the binding energy of
  the electron
• the remaining energy shows up as the kinetic
  energy of the electron

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Photoelectric effect

• Electron kinetic energy Photon energy - Binding
  energy
• Ekinetic(electron) h? - h?o
• Comments
• if frequency is too low, the photo energy is
  insufficient to overcome the binding energy of
  the electron
• energy in excess of the binding energy shows up
  as the kinetic energy of the electron
• increasing the intensity of the light increases
  the number of photons impacting on the metal

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Particle properties of light

• Light has a dual nature of acting like a wave and
  acting like a particle
• The photoelectric effect confirmed that light
  occurs as little packets of energy
• Light is still diffracted like a wave, has
  wavelength and frequency

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Light and atoms

• When matter absorbs photons of light, the energy
  of the photon is transferred to the matter
• In the case of atoms, the absorption process
  yields information about the atom
• Absorption of a photon transforms the atom to a
  higher energy state
• All higher energy states are referred to as
  excited states
• The most stable state is the ground state

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Absorption and Emission

• White light (light containing all energies of
  light) is passed through a sample
• Sample absorbs some of the light
• Light that passes through the sample is dispersed
  by a prism or other wavelength selecting device
• Photodetector records the intensity of the light
  passing through the sample, which is then
  interpreted as absorption of light

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Beers Law

• Io Intensity of incident light
• I Intensity of transmitted light
• ? molar extinction coefficient
• l path length of cell
• c concentration of sample

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UV Spectral Nomenclature
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UV and Visible Spectroscopy

• Vacuum UV or soft X-rays
• 100 - 200 nm
• Quartz, O2 and CO2 absorb strongly in this region
• N2 purge good down to 180 nm
• Quartz region
• 200 350 nm
• Source is D2 lamp
• Visible region
• 350 800 nm
• Source is tungsten lamp

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Emission

• Sample is excited by light
• Excited sample emits the light
• Emitted light is wavelength selected
• The light is detected by a photodetector
• Plot of emission intensity vs wavelength is
  generated

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Quantization of absorption and emission

• One of the three things that led to quantum
  theory was that the absorption and emission of
  light occurred at discrete frequencies, not
  continua
• Interpreted as the energy of the photon must
  match the difference in energy of two energy
  levels in the atom or molecule

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Molecular process

• Absorption and emission of visible and
  ultraviolet light
• Photon is annihilated upon absorption, and the
  electrons in the molecule are rearranged into the
  excited state
• Emission results from the conversion of excited
  electron energy being converted to a photon of
  light
• Ephoton ??Eatom?

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Energy level diagrams

• Wiggle lines indicate radiative processes
• Straight lines indicate nonradiative processes
• Each energy level represents an arrangement of
  electrons in the atom

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Properties of electrons

• Each electrons have the same mass and charge
• Electrons behave like magnets through a property
  called spin (actually, magnets are magnets
  because electrons have this property)
• Electrons have wave properties (diffract just
  like photons)

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Heisenberg uncertainty principle

• A particle has a particular location, but a wave
  has no exact position
• The wave properties of electrons cause them to
  spread out, hence the position of the electron
  cannot be precisely defined
• They are referred to as being delocalized in a
  region of space
• Heisenberg proposed that the motion and position
  of the particle-wave cannot be precisely known at
  the same time

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Bound electrons and quantization

• The properties of electrons bound to a nucleus
  can only take on certain specific values (most
  importantly, energy)
• Absorption and emission spectra provide
  experimental values for the quantized energies of
  atomic electrons
• Theory of quantum mechanics links these data to
  the wave characteristics of electrons bound to
  nuclei

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The Schrödinger Equation

• A second order partial differential equation
• The solutions to such equations are other
  equations
• These equations describe three-dimensional waves
  called orbitals
• These solutions have indexes that are integers
  (the solutions are quantized naturally)
• These indexes are called quantum numbers

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Quantum numbers

• n - principle quantum number
• values of the positive integers
• n 1,2,3,
• l - azimuthal quantum number
• values correlate with the number of preferred
  axes of a particular orbital, indicating its
  shape
• l 0,1,2,(n - 1)
• value of l is often indicated by a letter (s, p,
  d, f, for l 0, 1, 2, 3)

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Quantum number

• ml - magnetic quantum number
• directionality of orbital
• ml 0, 1, 2, l
• ms - spin orientation quantum number
• ms ½
• A complete description of an atomic electron
  requires a set of four unique quantum number that
  meet the restrictions of quantum mechanics

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Shapes of atomic orbitals

• Each atomic energy level can be associated with a
  specific three-dimensional atomic orbital
• Orbitals are maps of the probability of the
  electron being in a particular location around
  the nucleus
• While there are many representations, the most
  important to learn are the 90 probability
  volumes (which I will draw for you)

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Depictions of orbitals

• electron density plot - electron density plotted
  against the distance from the nucleus
• orbital density plots
• electron contour diagrams (90 probability
  drawings)
• All are useful in helping us visualize the orbital

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Waves and nodes
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A variety of radial projections
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Radial depictions
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The p-orbitals
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The d-orbitals
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d-orbital radial projection