Many electron atoms

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Electronic Structure of the Atom

• Many electron atoms
• An important factor affecting the energies of the
  orbital electrons in multi-electron atoms
• Effective nuclear charge the inner shell
  electrons reduce the nuclear charge felt by the
  outer electrons.
• The inner electrons partially cancel some of the
  positive charge of the nucleus to produce a
  screening effect.
• The higher the value for a given n value, the
  smaller the Zeffective
• The lower the value the higher the probability
  of finding an electron near the nucleus and
  the greater its Zeffective.
• Zeffective(3s electrons) gt Zeffective(3p
  electrons) gt Zeffective(3d electrons)
• For a given n value, the higher the value the
  higher the energy
• Electrons having the same value of n and have
  the same energy
• They are degenerate.
• See Fig. 8-8, Kotz Treichel for an example of
  these principles

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• Shielded and Zeffective for 3Li Atom
• (a) shows the charge felt by an electron in a
  Li atom as a function of distance from the
  nucleus
• Near the nucleus, the shielded charge is 3
• (b) shows that an electron in the 2s orbital is
  closer to the nucleus than an electron in a 2p
  orbital
• The electron in a 2s orbital has a higher
  shielded charge than that of an electron in a 2p
  orbital
• The energy of an electron in a 2s orbital is
  lower than that of an electron in a 2p orbital
• It takes more energy to remove an electron from
  a 2s orbital than from a 2p orbital

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Experimentally Determined Order of Shell and
Subshell Energies In a multielectron atom,
energies of electron shells increase with
increasing n, and subshell energies increase
with increasing . The energy axis is not to
scale. The energy gaps between subshells for a
given shell become smaller with increasing n.
Electrons are placed in orbitals in order of n
value. For two subshells with the same value
of n , electrons are assigned first to the
subshell with lower n value
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Energy
Approximate energy level diagram for atomic
orbitals in multielectron atoms This diagram
shows the order of orbital filling
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• Electronic Structure of Atoms
• Electron configurations
• The most stable arrangement of electrons has the
  electrons filling the lowest energy orbitals
  with no more than 2 electrons per orbital
• The auf bau principle (build up principle)
  generally allows predictions of how the
  electrons are arranged in the orbitals of an
  atoms in their ground state.

Hunds Rule of Maximum Spin Multiplicity the
lowest energy is obtained when electrons fill
degenerate orbitals to maximize the number of
unpaired spins.
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Electronic Structure of Atoms Electron
configurations
Table 8.2 in Kotz and Treichel gives the electron
configurations for the first 109 elements Your
VWR periodic table also gives the electron
configurations of the elements
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Electronic Structure of Atoms

• Some Exceptions to the auf bau principle
• Cr 1s22s22p63s23p64s13d5
• Cu 1s22s22p63s23p64s13d10
• Electron Configurations and the Periodic Table
• Group 1A Group 7A Group 4B
• Li He2s1 F He2s22p5
• Na Ne3s1 Cl Ne3s23p5
• K Ar4s1 Br Ar4s23d104p5 Ti Ar4s23d2
• Rb Kr5s1 I Kr5s24d105p5 Zr Kr5s24d2
• Cs Xe6s1 Hf Xe6s25d2
• The valence electrons are the electrons in an
  atom beyond the core electrons or the inert gas
  electrons of the electron configuration
• Length of the periods
• Period 1 has 2 elments Periods 4 and 5 have 18
  elements
• Periods 2 and 3 have 8 elements Periods 6 and 7
  have 32 elements

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Electronic Structure of Atoms

• Electron configuration of monatomic ions
• To form cations
• Remove one or more electrons from orbitals having
  the highest n value
• If there is a choice of subshells, remove
  electrons first from the subshell with the
  highest value
• Examples
• K 1s22s22p63s23p64s1 K
  1s22s22p63s23p6 1e- K Ar 1e-
• Al 1s22s22p63s23p1 Al3
  1s22s22p6 3e- Al3 Ne 3e-
• Cr 1s22s22p63s23p64s13d 5 Cr3
  1s22s22p63s23p63d 3 3e- Ar3d 3
• To form anions electrons are added to the highest
  energy orbital
• Example
• Cl 1s22s22p63s23p5 1e- Cl- 1s22s22p63s23p6

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Electronic Structure of Atoms

• Atomic Properties and the Periodic Table
• The similarities in the properties of the
  elements are a result of the similar atomic
  valence shell electron configurations
• We will examine trends in various physical and
  chemical properties as we move across the
  periodic table or down a column of the periodic
  table
• The properties well start with are
• Atomic size
• Ionization energy
• Electron affinity
• Monatomic ion size

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Periodic Properties of the Elements

• Atomic Size
• Atoms do not have a fixed size because the
  probability of finding the electron in the
  valence shell decreases with distance from the
  nucleus but never gets to zero.
• One measure of the size of an atom is its
  covalent radius measured from interatomic
  distances of atoms in molecules.
• The C-C distance in diamond and other compounds
  is 154 pm, rC77 pm
• The Cl-Cl distance in Cl2 is 199 pm, rCl99 pm
• In CCl4, rCCl177 pm which is close to 77 pm 99
  pm176 pm
• Metal atom radii measured from atom separations
  in metal crystals
• Periodic trends in atomic size of the
  representative elements
• In a vertical group, the lower the element the
  larger the atom
• In a horizontal period, moving to the right the
  smaller the atom

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Periodic Properties of the Elements

• These results are consistent with
• an increase in the radius of the valence shell
  with n
• Zeffective for the valence electrons is
  fairly constant as we move down a group
• an increase in Zeffective with atomic number for
  valence shell electrons having the same n, i.
  e., for atoms in the same period
• As we move right in a period, the nuclear
  charge increases but the number of inner shell
  electrons does not change
• Periodic trends in the size of the transition
  elements
• The trend in the size of these elements is
  somewhat different from the representative
  elements
• For the 1st few elements in a period, the atoms
  become smaller
• The elements in the middle of a period have small
  changes in size
• The last elements show slight increase in size
• The explanation involves considering the orbitals
  in which the electrons are being added
• The electrons are being added to the (n-1)d
  orbitals
• The size of the atom is determined by the ns
  electrons
• Increasing in the number of (n-1)d electrons
  increases d-d interelectron repulsion and
  cancels the increase in Zeffective

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