Ionic Bonds and Some MainGroup Chemistry

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Chapter 6

• Ionic Bonds and Some Main-Group Chemistry

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Ions and Their Electron Configurations

• Chapter 2
• Metals lose electrons to form cations
• Main group elements tend to lose the same of
  electrons as their group number
• Nonmetals when combined with metals gain
  electrons to form anions
• The number gained 8 group
• WHY???????

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Ions and Their Electron Configurations

• Main Group Elements - the number of electrons
  gained or lost the number of electrons
  necessary to obtain the nearest noble gas or
  pseudo-noble gas configuration (filled
  d-orbital).
• Main group metal the electrons lost often
  of valence electrons
• Nonmetal the electrons gained of
  electrons necessary to achieve 8 in the valence
  shell
• Na, Ne, Na O, Ne, O2-
• K, Ar, K Cl, Ar, Cl-
• Ga, Ga3 Sn, Sn4

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Ions and Their Electron Configurations

• Transition element ions formed by
• 1st losing their valence electrons
• 2nd losing d-orbital electrons to obtain the
  charge
• Fe2 Fe3 Co2 Co4
• Ag Zn2

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Problem

• Select the correct set of quantum numbers (n, l,
  ml, ms) for the first electron removed in the
  formation of a cation for strontium, Sr. 
• A.  5, 1 , 0, -½
• B.  5, 1, 0, ½
• C.  5, 0, 1, ½
• D.  5, 1, 1, ½
• E.  5, 0, 0, -½

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Ionic Radii

• Effect of charge on ionic radii evaluated using
  atomic size and Zeff
• Atomic size increases with increasing energy
  levels
• Zeff
• Cation more protons than electrons stronger
  attraction
• Anion less protons than electrons weaker
  attraction
• Cation lt Neutral atom lt anion

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Ionic Radii
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Ionization Energy

• Ionization Energy (Ei) the energy required by 1
  mole of gaseous atoms for 1 mole of electrons to
  be ejected.
• Na EI Na e-
• Energy necessary to make a cation
• More energy required for smaller atoms

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Ionization Energy
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Ionization Energy Trend

• Increases across a period held tighter
• Decreases down a group larger (n)

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Higher Ionization Energies

• Ionization energy not limited to a single
  electron
• 2nd electron removed requires even more energy
• Removal of first leads to more protons than
  electrons
• To break into a noble gas configuration requires
  a lot of energy because of their stability

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Problem

• Which one of the following equations correctly
  represents the process relating to the ionization
  energy of X? 
• A.  X(s) ? X(g) e-
• B.  X2(g) ? X(g) X-(g)
• C.  X(g) e- ? X-(g)
• D.  X-(g) ? X(g) e-
• E.  X(g) ? X(g) e-

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Problem

• Which of the following elements has the smallest
  first ionization energy? 
• A.  Rb
• B.  Mg
• C.  I
• D.  As
• E.  F

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Problem

• Which of the following elements has the largest
  second ionization energy (IE2)? 
• A.  Li
• B.  B
• C.  O
• D.  F
• E.  Na

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Electron Affinity

• Electron Affinity (Eea) - Describes the energy
  associated with the absorption of 1 mole of
  electrons by 1 mole of gaseous atoms
• Cl e- Cl-
• Can be positive or negative
• Positive energy had to be added to force the
  atom to absorb the electron (non-favored)
• Negative energy was given off when the electron
  was added to the atom (favored)

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Electron Affinity
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Electron Affinity

• Trend
• Eea becomes more negative across a period
• Eea becomes more positive down a group
• Larger atoms dont care as much

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Problem

• Which one of the following equations correctly
  represents the process involved in the electron
  affinity of X? 
• A.  X(g) ? X(g) e-
• B.  X(g) ? X(aq)
• C.  X(g) e- ? X(g)
• D.  X(g) e- ? X-(g)
• E.  X(g) Y-(g) ? XY(s)

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Problem

• Select the element with the most negative
  electron affinity (i.e., accepts an electron most
  readily). 
• A.  H
• B.  Li
• C.  C
• D.  F
• E.  Ne

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Ionic Bonds and the Formation of Ionic Solids

• Ionic bonds
• Form when an element with a small Ei value comes
  in contact with an element with a negative Eea
  value.

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Ionic Bonds and the Formation of Ionic Solids

• The energetics of ionic reactions can be viewed
  on a BornHaber Cycle which shows how each step
  contributes to the overall reaction energy.
• That energy is called the lattice energy (U) of
  the solid. By convention, the lattice energy
  refers to the breakup of the crystal into ions.

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Ionic Bonds and the Formation of Ionic Solids
BornHaber Cycle for NaCl
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Ionic Solids and the Formation of Ionic Compounds
BornHaber Cycle for MgCl2
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Ionic Bonds and the Formation of Ionic Solids

• Calculate the lattice energy (in kJ/mol) for the
  formation of CaH2 from its elements.
• Calculate the overall energy change (in kJ/mol)
  for the formation of CaCl from its elements.
• Calculate the overall energy change (in kJ/mol)
  for the formation of CaCl2 from its elements.
• Which is more likely to form, CaCl or CaCl2?

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Ionic Bonds and the Formation of Ionic Solids

• Trends in Lattice Energy
• Increases with increasing charge
• Increases with decreasing ion size

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The Octet Rule

• General Conclusions
• Group 1A form 1 by losing ns1 electron to obtain
  noble gas configuration
• Group 2A form 2 by losing ns2 electrons to
  obtain noble gas configuration
• Group 3A tend to form 3 by losing ns2np1
  electrons to obtain noble gas configuration
• Group 7A gain 1 electron to form ns2np6 hence
  giving them a noble gas configuration
• Group 8A neither gain nor lose electrons because
  of stability of configuration

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The Octet Rule

• Octet Rule main group elements tend to undergo
  reactions that leave them with 8 outer shell
  electrons
• Why does the rule work?

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The Octet Rule

• Electrons of metals are most likely lost due to
  the fact that
• core electrons are shielding them from the
  nucleus
• Zeff is lower
• Small ionization energies
• Upon loss they obtain noble gas configuration

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The Octet Rule

• Nonmetals are most likely to gain electrons due
  to the fact that
• no additional shielding occurs from the nucleus
  occurs due to core electrons
• Zeff is higher
• More negative electron affinities
• Upon gain they obtain noble gas configuration

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Optional Homework

• Text 6.30, 6.36, 6.38, 6.40, 6.42, 6.44, 6.46,
  6.48, 6.58, 6.64, 6.66
• Chapter 6 Homework

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Required Homework

• Assignment 6