SAT Subject Test Chemistry




Atomic Structure and the Periodic Table of the Elements


The elements involved with the filling of a d sublevel with electrons after two electrons are in the s sublevel of the next principal energy level are often referred to as the transition elements. The first examples of these are the elements between calcium, atomic number 20, and gallium, atomic number 31. Their electron configurations are the same in the 1s, 2s, 2p, 3s, and 3p sublevels. It is the filling of the 3d and changes in the 4s sublevels that are of interest, as shown in the following table.

The asterisk (*) shows where a 4s electron is promoted into the 3d sublevel. This is because the 3d and 4s sublevels are very close in energy and that there is a state of greater stability in half-filled and filled sublevels. Therefore, chromium gains stability by the movement of an electron from the 4s sublevel into the 3d sublevel to give a half-filled 3d sublevel. It then has one electron in each of the five orbitals of the 3d sublevel. In copper, the movement of one 4s electron into the 3d sublevel gives the 3d sublevel a completely filled configuration.

The fact that the electrons in the 3d and 4s sublevels are so close in energy levels leads to the possibility of some or all the 3d electrons being involved in chemical bonding. With the variable number of electrons available for bonding, it is not surprising that transition elements can exhibit variable oxidation numbers. An example is manganese with possible oxidation numbers of +2, +3, +4, +6, and +7, which correspond, respectively, to the use of no, one, two, four, and five electrons from the 3d sublevel.

The transition elements in the other periods of the table show this same type of anomaly, as they have d sublevels filling in the same manner.

Transition elements have several common characteristic properties.

• They often form colored compounds.

• They can have a variety of oxidation states.

• At least one of their compounds has an incomplete d electron subshell.

• They are often good catalysts.

• They are silvery blue at room temperature (except copper and gold).

• They are solids at room temperature (except mercury).

• They form complex ions.

• They are often paramagnetic.