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• 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.
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?