POLYELECTRONIC ATOMS PERIODICITY OF ELEMENTS Part 2 Sec 913

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POLYELECTRONIC ATOMSPERIODICITY OF
ELEMENTS(Part 2 Sec 9-13)

• Electronic Configurations
• Periodic Trends

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POLYELECTRONIC ATOMS

• We will extend the 1-electron results (energy
  eqn, quantum numbers, AO shapes, etc) to the
  polyelectronic atom (atoms with gt 1 electron).
• Electrostatic forces in the poly-e atom include
  nucleus-electron attractions and
  electron-electron repulsions.
• The e-e repulsions prevent the exact solution of
  the Schrodinger Eqn for helium and above.
  (Electron Correlation Problem)
• The presence of gt 1 electron results in partial
  shielding of the nuclear charge. I.e. Zeff
  Zactual electron shielding

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POLYELECTRONIC ATOMS (2)

• Also, because of the difference in AO shape for
  different l values, electrons with the same n but
  in different subshells (l) experience different
  attractive forces to the nucleus.
• As the electron spend more time near the nucleus,
  its energy becomes more negative (is held more
  tightly).

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POLYELECTRONIC ATOMS (3)

• For the 1-e atom, AO energy depends only on n.
  As n increases, energy increases (becomes less
  positive). So AOs can be ordered from low to
  high energy 1s lt 2s, 2p lt 3s, 3p, 3d... Fig
  7.18 (note degeneracy).
• For the many-electron atom, energy depends on n
  and l 1s lt 2s lt 2p lt 3s lt 3p, etc. See Fig
  7.22 (note that the degeneracy is partially
  lifted).

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Figure 7.22 The Orders of the Energies of the
Orbitals in the First Three Levels of
Polyelectronic Atoms
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PERIODIC TABLE

• Most powerful tool in the study of chemistry.
• Elements are placed in order by Z.
• Used to predict undiscovered elements and their
  properties. Tables 7.3, 7.4
• Quantum mechanics explains the appearance of the
  PT and also periodicity of atomic properties

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Table 7.3 Comparison of the Properties of
Germanium as Predicted by Mendeleev and as
Actually Observed
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Table 7.4 Predicted Properties of Elements 113
and 114
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From H to Polyelectronic Atoms

• In H, AOs with the same n have the same E.
• In multielectron atoms,
• For a given set of QNs, orbital energies are
  lower than in H (larger Z value).
• AOs with the same n value but different l have
  different energies (nondegenerate).
• orbitals with the same n and l have the same E.
• 3d and 4s have similar energies

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DETERMINING ELECTRONIC CONFIGURATIONS

• What is the arrangement of electrons in the atom?
  What rules govern these arrangements or
  electronic configurations?
• There are four quantum numbers n, l, ml, ms
  that have defined relationships to each other and
  that are used to define a set of atomic orbitals
  (AOs) that electrons fill.

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ELECTRON CONFIG. (2)

• Aufbau (building-up) Principle determines the
  order of filling AOs i.e. the electronic
  configuration of the atom. The electrons fill
  the AOs in order of lowest energy (most negative)
  to highest energy.
• The atoms ground state is the one that has the
  lowest energy. All others are called excited
  states.

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ELECTRON CONFIG. (3)

• Pauli Exclusion Principle The maximum number of
  electrons per orbital is 2 because no two
  electrons can have the same 4 q.n. values in an
  atom. Since there are two spin states (up/down
  a/ß ?/?), the max AO occupancy number is 2.

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ELECTRON CONFIG. (4)

• Hunds Rule When filling orbitals of identical
  energy, fill the empty orbitals with one electron
  before pairing them up I.e. maximize number of
  unpaired spins. This produces the lowest energy
  configuration.
• There are exceptions e.g. 4s fill before 3d
  Cr, Cu have a single 4s electron and fully- or
  half-filled 3d orbitals (extra stability).
• Electron configs. of cations (remove from largest
  n AO) and anions (add according to Aufbau Prin.)

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ELECTRON CONFIG. (5)

• Ways to depict electronic config
• AO list with electrons as superscripts
• orbital diagram
• noble gas core for core electrons valence
  electrons (VE). Fig 7.25
• Valence electrons are the outermost electrons and
  the most important ones in chemical bonding.

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ELECTRON CONFIG. (6)

• Atoms in the same group have the same VE
  configuration.
• Learn electron config.s through Kr use PT to
  identify VEs of atoms beyond Kr.

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ELECTRON CONFIG. (7)

• Fig 7.27 shows how the Periodic Table can be used
  to determine electron config.
• Main Group s- and p- block elements
• Transition Group d-block elements
• Lanthanide and Actinide f-block element

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Figure 7.27 The Orbitals Being Filled for
Elements in Various Parts of the Periodic Table
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Figure 7.28 The Periodic Table with Atomic
Symbols, Atomic Numbers, and Partial Electron
Configurations
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PERIODIC LAW

• Now we can see how the electronic configurations
  of atoms lead to periodicity of elements and form
  the basis for chemical and physical properties of
  elements.
• PERIODIC LAW When the elements are arranged
  according to Z, their physical and chemical
  properties vary periodically, regularly and
  predictably.

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PERIODIC LAW (2)

• Elements in the nth A-Group (Main Group) have n
  electrons in the valence (outermost occupied)
  shell.
• Elements in the nth period have n as the
  principal QN of its valence shell.

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PERIODICITY OF ATOMIC PROPERTIES

• Ionization Energy energy required to remove one
  electron from gaseous atom or ion (kJ/mol)
• Fig 7.7.30, 7.31 Table 7.5, 7.6
• Atom (g) -----gt Ion(g) electron, I1
• Across row, I1 increases WHY?
• Down group, I1 decreases WHY?
• I1 is highest in upper RH corner

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Figure 7.30 The Values of First Ionization
Energy for the Elements in the First Six Periods
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Table 7.5 Successive Ionization Energies in
Kilojoules per Mole for the Elements in Period 3
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PERIODICITY OF ATOMIC PROPERTIES (2)

• Electron Affinity energy associated with
  addition of electron to gaseous species (kJ/mol)
• Fig 7.32, Table 7.7
• Atom (g) electron -----gt Ion- (g)
• Note most EA values are negative (exothermic)
• Trend across row and down group? And WHY?
• EA is highest in upper RH corner

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Figure 7.32 The Electron Affinity Values for
Atoms Among the First 20 Elements that Form
Stable, Isolated X- Ions
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PERIODICITY OF ATOMIC PROPERTIES (3)

• Atomic Radius measure of atomic size
• Atomic radii based on covalent or metallic bond
  distances (pm)
• Figs 7.34
• Trend across row and down group? And WHY?
• Atomic size is largest in lower LH corner
• Note that size of cation lt size of neutral atom
• But size of anion gt size of neutral atom

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Figure 7.34 Atomic Radii (in Picometers) for
Selected Atoms
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PERIODICITY OF ATOMIC PROPERTIES (4)

• Metallic Character
• Form cations, small I
• lower LH corner
• alkali metals
• Nonmetallic Character
• Form anions, large I
• upper RH corner