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 due to unpaired electrons.PERIODIC TABLE OF THE ELEMENTS
History
The history of the development of a systematic pattern for the elements includes
the work of a number of scientists such as John Newlands, who, in 1863,
proposed the idea of repeating octaves of properties.
TIP
Mendeleev is given credit for the first Periodic Table. It was based on placement by
properties.Dimitri I. Mendeleev in 1869 proposed a table containing 17 columns and
is usually given credit for the first periodic table since he arranged elements in
groups according to their atomic weights and properties. It is interesting to note