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-