Chemical Bonding
Chapter 8
Polarity and Dipole moments
Review
• Order the following
bonds according to
polarity: H-H, O-H,
Cl-H, S-H, and F-H.
• H-H
• S-H
• Cl-H
• O-H
• F-H
Bond Polarity and Dipole Moments
• Molecules with a
charge distribution of
a positive end and a
negative end are
dipolar, or said to
have a dipole
moment.
• But what about
molecules made up of
more than one atom?
Polar molecules are not always
dipoles
• In a linear molecule
such as carbon
dioxide, the individual
bond polarities are
arranged in such a
way that they cancel
each other out.
• Water is a bent
molecule with a
permanent dipole.
Other molecules with polar bonds
but no dipole.
Electron Configurations
• Electron arrangement
helps us to
understand
configurations of
compounds.
• In stable compounds,
virtually every atom
has a noble gas type
arrangement of
electrons.
Generalizations of electrons in
stable compounds
• Two non-metals react to form covalent
bonds in a way that completes the valence
electron configuration of both atoms.
• A metal and a non-metal react to form a
binary ionic compound. The ions form to
achieve the electron configuration of the
nearest noble gas.
Predicting formation of ions
• When discussing
ionic compounds,
scientists are
generally referring to
ions in their solid
state, not gaseous
state.
Predicting ions continued
• Atoms lose or gain
electrons to imitate
the nearest noble
gas. This gain or loss
of electrons results in
a charged atom called
an ion.
• Ions are attracted to
oppositely charged
ions and bond to form
neutral compounds
Exceptions
•
•
•
•
Elements in Group 1A lose an electron.
Elements in Group 2A lose 2 electrons.
Elements in Group 7A gain an electron.
Elements in Group 6A gain 2 electrons,
etc.
• But…elements Sn may lose 2 or 4
electrons.
• Pb2+ or Pb4+, Bi 3+ or Bi5+, Cu1+ or Cu2+
and so on.
Size and Charge
• Ion size is important
in determining the
structure and stability
of ionic solids.
• What determines the
Size?
• Look first at relative
size of ion and its
parent atom.
Ion Size
• Positively charge ions
have lost outer shell
electrons and are
smaller than their
parent atom.
• Negatively charged
ions have gained
electrons and are
larger than their
parent atom.
Ion Size
Isoelectronic ions
• These are ions of
different elements
with the same number
of electrons.
• For example O2-, F-,
Na+, Mg2+ and Al3+
• All have the Electron
configuration of Neon.
• What is Z?
Examples
• Arrange the ions in
order of decreasing
size.
• Se2-, Br-, Rb+ and Sr2+
• Se2• Br• Rb+
• Sr2+
• Choose the largest in
each group.
• Li+, Na+, K+, Rb+,
Cs+
• Answer: Cs+
• Ba2+, Cs+, I-, Te2• Te2-