Ionic bonds and some maingroup chemistry
Chapter 6
Ions and their electron configurations
• Metals – tend to give up electrons, coming
from the highest-energy occupied orbital.
Sodium atom (1s22s22p63s1) reacts with
chlorine and gives up an electron from the 3s,
giving an ion with noble gas electron
configuration of neon (1s22s22p6)
• Nonmetals – tend to accept electrons, fill
vacancy in 3p subshell to give electron
configuration of argon (1s22s22p63s23p6)
Problem: Ion configuration
Write the ground state configuration for Mg.
Write the ion configuration for Mg.
Write the ground state configuration for O.
Write the ion configuration for O.
Transitional metal ions
• As usual the transitional metals are different.
They react with nonmetals to lose their 4s
orbital 1st and then their 3d electrons. Ion
formation is different than the building up
process, so can’t be “reversed.” In building up
also adding a proton / positive charge to
nucleus.
Ionic radii
• Cation radii are smaller than their neutral
atoms, both because their principal quantum
number of the valence – shell is smaller for
cations and because Zeff (effective nuclear
charge) is larger from decrease in electrons.
• Anions radii are larger than their neutral
atoms because of additional electron-electron
repulsions and a decrease of Zeff from the
additional electrons.
Problem: Radii comparison
• Which atom or ion in each of the following
pairs would you expect to be larger?
• O or O-2
• O or S
• Fe or Fe+3
Ionizing energy
• Ei is the amount of energy needed to remove
the highest energy electron from an isolated
neutral atom in the gaseous state, for
hydrogen it’s 1312 kJ/mol.
• Ei decrease going down a group, goes up as
move to right. Slight decrease at new p (main
group 3), beginning new filled orbital (main
group 6)
Problem: ionization energies
• Arrange the elements Se, Cl and S in order of
increasing ionization energy.
• Predict which element has the larger
ionization energy.
K or Br
S or Te
Higher ionization energies
• Ionization is not limited to the loss of a single
electron from an atom. Two, three or more
electrons can be lost sequentially from an
atom.
• M + energy  M+ + e- 1st ionization energy
• M+ + energy  M+2 + e- 2nd ionization energy
• M+2 + energy  M+3 +e- 3rd ionization energy
• It takes larger amount of energy to remove
more electrons than previous due to stronger
pull of positively charge ion.
Problem: higher ionization energies
• What do you notice on the ionization chart as
you get to a filled valence shell (s2p6)?
• Which has the larger 5th ionization energy, Ge
or As?
• Which has the larger 3rd ionization energy, Be
or N?
Electron Affinity
• The measure of energy change on adding an
electron to an atom to form an anion.
Ionization energies are always positive
because energy is added to remove an
electron (absorbed, endothermic). Electron
affinities (Eea)generally negative because
energy is released when a neutral atom adds
an electron (released, exothermic).
Electron Affinity chart (- kJ/mol)
Trend in Eea
• Halogens have large negative Eea for both high
Zeff and room in shell.
• Positive Eea are found for noble gas because
the s and p are full.
• Near zero Eea for alkaline earth metals due to
filled s subshell.
Problem: electron affinity
• Why does nitrogen has a less favorable (more
positive) Eea than its neighbors on either side,
C and O?
Ionic bonds and the formation of ionic
solids
• A tale of 2 elements: sodium and chlorine.
Sodium and chlorine meet and sodium gives its
electron to chlorine. Sodium forms a cation and
chlorine forms an anion. They are attracted to
one another by electrostatic forces and are joined
by an ionic bond. The crystalline substance
formed is an ionic solid, due to their communal
living (sodium surrounded by other chlorines, and
chlorine surrounded by other sodium)
Energy changes between sodium and
chlorine: Born-Haber cycle
• Step 1: The conversion of solid Na metal into
isolated, gaseous Na atoms, a process called
sublimation. (+108 kJ/mol)
• Step 2: The dissociation of gaseous Cl2 molecules
into individual Cl atoms. Energy added to break
appart. (+122 kJ/mol)
• Step 3: The ionization of the isolated Na atoms
into Na+ ions (+ 496 kJ/mol)
• Step 4: The formation of Cl- ion (-349 kJ/mol)
• Step 5: The formation of solid NaCl from isolated
Na+ and Cl- (-788)
Born-Haber diagram for NaCl
Problem: Net energy calculations
• Calculate the net energy change for the
formation of KF(s) from the elements
K(s) + ½ F2(g)  KF (s)
• Heat of sub K = 89.2 kJ/mol
• Bond dissocition F2= 158 kJ/ mol
• Electrostatic interaction in KF = -821 kJ/mol
• Eea for F = -328 kJ/mol
• Ei for K = 418.8 kJ/mol
Lattice energies in ionic solids
• The measure of electrostatic interaction energies
between ion in a crystal and the measure of the
strength of a crystal’s ionic bonds.
• The force results is described by Coulomb’s law
and is equal to the constant k times the charges
on the ion, z1,z2 divided by the square of the
distance between centers.
• -U =F=k X (z1z2) / d2 Coulomb’s law
• -U is the lattice energy
Lattice energy
Problem: lattice energies
• Which has the larger lattice energy, NaCl or
CsI?
The octet rule
• Main group elements tend to undergo
reactions that leave them with eight outershell electrons, a noble gas electron
configuration with filled s and p sublevels in
their valence shells.
• Exceptions occur toward the right side of the
periodic table (groups 3-8) in the 3 period and
below (NCl3 vs PCl5)
Problem: chemical reactions and the
octet rule
• Lithium metal reacts with nitrogen to yield
Li3N. What noble gas configuration does the
nitrogen atom in Li3N have?
End section 6a
Chemistry of group 1A elements: Alkali
metals
Chemistry of group 2A elements:
alkaline earth metals
Chemistry of the Group 3A elements:
Aluminum
Chemistry of the Group 7A Elements:
Halogens
Chemistry of the group 8A elements:
Noble gases