|CH301H - Principles
of Chemistry I: Honors
Fall 2011, Unique 51040
Lecture Summary, 1 September 2011
|Periodic Trends Continued: Today we talked about electron
affinity, which is the energy gained by adding an electron to an atom
to make a negatively charged ion, or anion. We saw that the
periodic trend for electron affinity is roughly the same as for
ionization energy: the magnitude of energy gain increases across a
period and decreases down a group. Therefore, the atoms that give
up an electron most easily (group I) are also that atoms that are most
difficult to make accept an electron, and vice versa. It is very
important that you are comfortable with the signs of IE and EA.
Although IE and EA are properties of individual atoms, they influence how an atom will behave when it is involved in a bonding interaction with another atom. The physical properties of the two bonded atoms will give the molecule physical properties that we would like understand and predict. For example, an atom that has a high ionization energy and high electron affinity will tend to sequester the shared electrons in the bond, making the bond polar or even ionic. This is such an important concept that chemists have put the information contained in atomic IE and EA together into the concept of electronegativity. EN is a purely empirical concept; i.e. it is based only on observation and experience, not on logic, and cannot be derived from first principles. It is not a physical quantity that can be measured, which should be clear to you from the discussion in your book on how Mulliken and Pauling established their EN scales. Like IE and EA, EN increases across a period and decreases down a group.
Ionic Bonding: When an atom with low IE and low EA bonds with an atom with high IE and high EA, such as group I or II bonding with group VII, the high EN atom will completely sequester the shared electrons, on the bond will be ionic. The strength of an ionic bond can be determined by tabulating the energy needed to form each ion and the electrostatic potential energy gained by allowing the ions to interact. We did this for the reaction
K + F --> KF
and were able to draw the potential energy curve for the stable KF molecule.