The d-electron elements have distinctive physical properties that make them useful as structural metals and catalysts, and distinctive electronic properties that find applications in the molecules of living organisms. The physical properties will be discussed in this section, and the electronic properties in the next.

The four series of transition metals, and the electron-filling process for the first of these, are shown below. Generally, each new transition metal along one row has two electrons in the outer s orbital, and an increasing number of electrons in the buried d orbital belonging to the preceding principal quantum number. In Row 4 chromium (Cr) and copper (Cu) are exceptions to this rule, stealing one of the two s electrons to half-fill or fill the d orbitals. These are minor exceptions, and it is the filling principle that is important.

In chemical reactions of the transition metals, the s electrons are lost most easily, and +1 and +2 oxidation states are common. Higher oxidation states also are possible for atoms that have d electrons and, in principle, the highest possible state would correspond to the loss of all of the outer s and d electrons: +3 for scandium, +4 for titanium, and +7 for manganese. Electron pairing complicates matters from iron onward, as we shall see.