Chapter 6 Atomic Structure

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Chapter 6Atomic Structure
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ATOMIC STRUCTURE
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ELECTROMAGNETIC RADIATION
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Electromagnetic Radiation

• Most subatomic particles behave as PARTICLES and
  obey the physics of waves.

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Electromagnetic Radiation
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Electromagnetic Radiation
See Figure 6.1
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Electromagnetic Radiation

• Waves have a frequency
• Use the Greek letter nu, ?, for frequency, and
  units are cycles per sec
• All radiation ? ? c
• c velocity of light 3.00 x 108 m/sec
• Long wavelength f small frequency
• Short wavelength f high frequency

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Electromagnetic Radiation

• Long wavelength f small frequency
• Short wavelength f high frequency

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Electromagnetic Radiation

• Red light has ? 700 nm. Calculate the
  frequency.

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Electromagnetic Radiation
Short wavelength f high frequency high energy

• Long wavelength f
• small frequency
• low energy

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Electromagnetic Spectrum
See Active Figure 6.2
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Quantization of Energy
Max Planck (1858-1947) Solved the ultraviolet
catastrophe
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See Chem Chem Reactivity, Figure 6.3
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Quantization of Energy

• An object can gain or lose energy by absorbing or
  emitting radiant energy in QUANTA.

• Energy of radiation is proportional to frequency

E h ?
h Plancks constant 6.6262 x 10-34 Js
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Quantization of Energy
E h ?
Light with large ? (small ?) has a small E.
Light with a short ? (large ?) has a large E.
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Photoelectric Effect
Experiment demonstrates the particle nature of
light.
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Photoelectric Effect

• Classical theory said that E of ejected electron
  should increase with increase in light
  intensitynot observed!
• No e- observed until light of a certain minimum E
  is used.
• Number of e- ejected depends on light intensity.

A. Einstein (1879-1955)
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Photoelectric Effect

• Understand experimental observations if light
  consists of particles called PHOTONS of discrete
  energy.

PROBLEM Calculate the energy of 1.00 mol of
photons of red light. ? 700. nm ? 4.29 x
1014 sec-1
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Energy of Radiation

• Energy of 1.00 mol of photons of red light.
• E h?
• (6.63 x 10-34 Js)(4.29 x 1014 s-1)
• 2.85 x 10-19 J per photon
• E per mol
• (2.85 x 10-19 J/ph)(6.02 x 1023 ph/mol)
• 172 kJ/mol
• This is in the range of energies that can break
  bonds.

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Excited Atoms Atomic Structure
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Atomic Line Emission Spectra and Niels Bohr

• Bohrs greatest contribution to science was in
  building a simple model of the atom. It was based
  on an understanding of the SHARP LINE EMISSION
  SPECTRA of excited atoms.

Niels Bohr (1885-1962)
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Spectrum of White Light
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Line Emission Spectra of Excited Atoms

• Excited atoms emit light of only certain
  wavelengths
• The wavelengths of emitted light depend on the
  element.

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Spectrum of Excited Hydrogen Gas
See Active Figure 6.6
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Line Emission Spectra of Excited Atoms
High E Short ? High ?
Low E Long ? Low ?

• Visible lines in H atom spectrum are called the
  BALMER series.

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Line Spectra of Other Elements
See Figure 6.7
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The Electric Pickle
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• Excited atoms can emit light.
• Here the solution in a pickle is excited
  electrically. The Na ions in the pickle juice
  give off light characteristic of that element.

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Atomic Spectra and Bohr
One view of atomic structure in early 20th
century was that an electron (e-) traveled about
the nucleus in an orbit.

• 1. Any orbit should be possible and so is any
  energy.
• 2. But a charged particle moving in an electric
  field should emit energy.
• End result should be destruction!

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Atomic Spectra and Bohr

• Bohr said classical view is wrong.
• Need a new theory now called QUANTUM or WAVE
  MECHANICS.
• e- can only exist in certain discrete orbits
  called stationary states.
• e- is restricted to QUANTIZED energy states.
• Energy of state - C/n2
• where n quantum no. 1, 2, 3, 4, ....

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Atomic Spectra and Bohr
Energy of quantized state - C/n2

• Only orbits where n integral no. are permitted.
• Radius of allowed orbitals n2 (0.0529 nm)
• But note same eqns. come from modern wave
  mechanics approach.
• Results can be used to explain atomic spectra.

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Atomic Spectra and Bohr

• If e-s are in quantized energy states, then ?E
  of states can have only certain values. This
  explain sharp line spectra.

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Energy Adsorption/Emission
See Active Figure 6.9
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Atomic Spectra and Bohr

• Calculate ?E for e- falling from high energy
  level (n 2) to low energy level (n 1).
• ?E Efinal - Einitial -C(1/12) - (1/2)2
• ?E -(3/4)C
• Note that the process is EXOTHERMIC

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Atomic Spectra and Bohr

• ?E -(3/4)C
• C has been found from experiment (and is now
  called R, the Rydberg constant)
• R ( C) 1312 kJ/mol or 3.29 x 1015 cycles/sec
• so, E of emitted light
• (3/4)R 2.47 x
  1015 sec-1
• and l c/n 121.6 nm
• This is exactly in agreement with experiment!

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Origin of Line Spectra
Balmer series
See Active Figure 6.10
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Atomic Line Spectra and Niels Bohr

• Bohrs theory was a great accomplishment.
• Recd Nobel Prize, 1922
• Problems with theory
• theory only successful for H.
• introduced quantum idea artificially.
• So, we go on to QUANTUM or WAVE MECHANICS

Niels Bohr (1885-1962)
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Quantum or Wave Mechanics

• de Broglie (1924) proposed that all moving
  objects have wave properties.
• For light E mc2
• E h? hc / ?
• Therefore, mc h / ?
• and for particles
• (mass)(velocity) h / ?

L. de Broglie (1892-1987)
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Quantum or Wave Mechanics

• Baseball (115 g) at 100 mph
• ? 1.3 x 10-32 cm
• e- with velocity
• 1.9 x 108 cm/sec
• ? 0.388 nm

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Experimental proof of wave properties of electrons
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Quantum or Wave Mechanics

• Schrodinger applied idea of e- behaving as a wave
  to the problem of electrons in atoms.
• He developed the WAVE EQUATION
• Solution gives set of math expressions called
  WAVE FUNCTIONS, ?
• Each describes an allowed energy state of an e-
• Quantization introduced naturally.

E. Schrodinger 1887-1961
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Wave motion wave length and nodesQuantization
in a standing wave
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WAVE FUNCTIONS, ?

• ??is a function of distance and two angles.
• Each ? corresponds to an ORBITAL the region
  of space within which an electron is found.
• ? does NOT describe the exact location of the
  electron.
• ?2 is proportional to the probability of
  finding an e- at a given point.

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Uncertainty Principle

• Problem of defining nature of electrons in atoms
  solved by W. Heisenberg.
• Cannot simultaneously define the position and
  momentum ( mv) of an electron.
• We define e- energy exactly but accept limitation
  that we do not know exact position.

W. Heisenberg 1901-1976
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Types of Orbitals
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s orbital
p orbital
d orbital
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Orbitals

• No more than 2 e- assigned to an orbital
• Orbitals grouped in s, p, d (and f) subshells

s orbitals
p orbitals
d orbitals
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s orbitals
p orbitals
d orbitals
No. orbs.
1
3
5
No. e-
2
6
10
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Subshells Shells

• Subshells grouped in shells.
• Each shell has a number called the PRINCIPAL
  QUANTUM NUMBER, n
• The principal quantum number of the shell is the
  number of the period or row of the periodic table
  where that shell begins.

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Subshells Shells
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QUANTUM NUMBERS

• The shape, size, and energy of each orbital is a
  function of 3 quantum numbers
• n (major) f shell
• s (angular) f subshell
• ms (magnetic) f designates an orbital
  within a subshell

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QUANTUM NUMBERS

• Symbol Values Description
• n (major) 1, 2, 3, .. Orbital size
  and energy where E -R(1/n2)
• s (angular) 0, 1, 2, .. n-1 Orbital shape
  or type (subshell)
• ms (magnetic) -s..0..s Orbital
  orientation
• of orbitals in subshell
  2s 1

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Types of Atomic Orbitals
See Active Figure 6.14
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Shells and Subshells

• When n 1, then s 0 and ms 0
• Therefore, in n 1, there is 1 type of subshell
• and that subshell has a single orbital
• (ms has a single value f 1 orbital)
• This subshell is labeled s (ess)
• Each shell has 1 orbital labeled s, and it is
  SPHERICAL in shape.

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s Orbitals Always Spherical

• Dot picture of electron cloud in 1s orbital.

Surface density 4pr2y versus distance
Surface of 90 probability sphere
See Active Figure 6.13
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1s Orbital
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2s Orbital
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3s Orbital
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p Orbitals

• When n 2, then s 0 and 1
• Therefore, in n 2 shell there are 2 types of
  orbitals 2 subshells
• For s 0 ms 0
• this is a s subshell
• For s 1 ms -1, 0, 1
• this is a p subshell with 3 orbitals

When s 1, there is a PLANAR NODE thru the
nucleus.
See Screen 6.15
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p Orbitals

• The three p orbitals lie 90o apart in space

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2px Orbital
3px Orbital
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d Orbitals

• When n 3, what are the values of s?
• s 0, 1, 2
• and so there are 3 subshells in the shell.
• For s 0, ms 0
• f s subshell with single orbital
• For s 1, ms -1, 0, 1
• f p subshell with 3 orbitals
• For s 2, ms -2, -1, 0, 1, 2
• f d subshell with 5 orbitals

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d Orbitals

• s orbitals have no planar node (s 0) and so are
  spherical.
• p orbitals have s 1, and have 1 planar node,
• and so are dumbbell shaped.
• This means d orbitals (with s 2) have 2 planar
  nodes

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3dxy Orbital
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3dxz Orbital
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3dyz Orbital
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3dx2- y2 Orbital
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3dz2 Orbital
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f Orbitals

• When n 4, s 0, 1, 2, 3 so there are 4
  subshells in the shell.
• For s 0, ms 0
• f s subshell with single orbital
• For s 1, ms -1, 0, 1
• f p subshell with 3 orbitals
• For s 2, ms -2, -1, 0, 1, 2
• f d subshell with 5 orbitals
• For s 3, ms -3, -2, -1, 0, 1, 2, 3
• f f subshell with 7 orbitals

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f Orbitals

• One of 7 possible f orbitals.
• All have 3 planar surfaces.
• Can you find the 3 surfaces here?

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Spherical Nodes
2 s orbital

• Orbitals also have spherical nodes
• Number of spherical nodes n - s - 1
• For a 2s orbital No. of nodes 2 - 0 - 1
  1

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Arrangement of Electrons in Atoms

• Electrons in atoms are arranged as
• SHELLS (n)
• SUBSHELLS (s)
• ORBITALS (ms)

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Arrangement of Electrons in Atoms

• Each orbital can be assigned no more than 2
  electrons!
• This is tied to the existence of a 4th quantum
  number, the electron spin quantum number, ms.

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Electron Spin Quantum Number, ms
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Can be proved experimentally that electron has an
intrinsic property referred to as spin. Two
spin directions are given by ms where ms 1/2
and -1/2.
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Electron Spin and Magnetism

• Diamagnetic NOT attracted to a magnetic field
• Paramagnetic substance is attracted to a
  magnetic field.
• Substances with unpaired electrons are
  paramagnetic.

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Measuring Paramagnetism
Paramagnetic substance is attracted to a
magnetic field. Substance has unpaired
electrons. Diamagnetic NOT attracted to a
magnetic field
See Active Figure 6.18
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QUANTUM NUMBERS
Now there are four!

• n f shell 1, 2, 3, 4, ...
• s f subshell 0, 1, 2, ... n - 1
• ms f orbital - s ... 0 ... s
• ms f electron spin 1/2 and -1/2